International Neutrality Law and U.S. Military Assistance to Ukraine
Legal Sidebari
International Neutrality Law and U.S.
Military Assistance to Ukraine
April 26, 2022
The United States, the European Union (EU), and others have supplied many forms of security assistance
to Ukraine in the weeks since Russia’s invasion. Recent U.S. assistance to Ukraine, discussed in an earlier
In Focus, ranges from ammunition to anti-aircraft weapons to communications systems. At the same time,
the United States has stopped short of sending some military equipment requested by Ukrainian President
Volodymyr Zelensky, such as combat aircraft. Deciding which arms to provide raises a variety of legal,
political, and practical considerations, including the potential for escalation with Russia, the Ukrainian
military’s capacity to operate the equipment, and the risk that Russia could reverse engineer captured
equipment. While international law is just one facet of this calculus, media outlets report that the Biden
Administration discussed questions about the legality of U.S. security assistance, and observers have
analyzed whether supplying arms could violate the international law of neutrality.
International neutrality law governs the legal relationship between countries that are not taking part in an
international armed conflict (neutral states) and those that are engaged in such a conflict (belligerents).
The international community developed the principles of the international law of neutrality in an era
before the Charter of the United Nations (U.N.) prohibited using force as a tool to resolve international
conflict. Scholars have described the law of neutrality as an “old body of law” with a “slightly musty
quality” that does not always translate to modern warfare.
Russia and Ukraine are engaged in an international armed conflict and, thus, are belligerents. Under
traditional conceptions of neutrality, sending “war material of any kind” to Ukraine or any other
belligerent would violate a duty of neutrality; however, some countries, including the United States, have
adopted the doctrine of qualified neutrality. Under this doctrine, states can take non-neutral acts when
supporting the victim of an unlawful war of aggression. For the reasons discussed in an earlier Sidebar,
Ukraine has firm grounds to contend that it is such a victim and is acting in self-defense. Under these
circumstances, arms assistance to Ukraine would generally be lawful under the qualified neutrality
doctrine, provided that Ukraine complies with other legal frameworks governing the conduct of
hostilities.
Even if qualified neutrality did not apply in this instance and U.S. security assistance breached a duty of
neutrality, international law would limit the breach’s legal consequences. For example, security assistance
to Ukraine would not permit Russia to use force against the United States in response to a neutrality
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violation unless Russia could satisfy an exception to the U.N. Charter’s prohibition on use of force. Nor
would a violation of neutrality, on its own accord, make the United States a co-belligerent or party to the
conflict fighting alongside Ukraine. Questions of co-belligerency implicate other legal paradigms and are
not resolved by neutrality law alone.
This Legal Sidebar discusses international neutrality law and its relationship with U.S. security assistance
to Ukraine. (Another CRS In Focus discusses the domestic laws concerning neutrality, including
restrictions on U.S. nationals serving in a foreign military.)
Sources and Requirements of the Law of Neutrality
The law of neutrality has its roots in 17th and 18th century state practice in which countries developed a
system of reciprocal rights and obligations for neutral states and belligerents. Neutral states have a duty
not to participate in hostilities and to be impartial in their conduct toward belligerents. In return,
belligerents are obligated to respect neutral states’ territory, and neutrals are permitted to trade with all
sides of the conflict if they do so in an impartial way. Countries eventually came to accept certain
principles of neutrality as part of customary international law—a body of law that is derived from state
practice followed out of a sense of legal obligation.
Many facets of neutrality law were defined in two treaties adopted at a 1907 peace conference:
the Hague Convention (V) Respecting the Rights and Duties of Neutral Powers and
Persons in Case of War on Land (Hague V) and
the Hague Convention (XIII) Respecting the Rights and Duties of Neutral Powers in
Naval War (Hague XIII).
Under Hague V and XIII, neutral states cannot provide “ammunition, or war material of any kind
whatever” to belligerents. The treaties exempt humanitarian assistance from this prohibition, and they do
not require neutral states to prevent private companies from selling munitions and war material. Neutral
states also have an obligation to prevent belligerents from committing certain hostile acts on neutral
states’ territory, and Hague V and XIII require neutrals to intern and detain belligerent forces found in
their territory. As part of their corresponding set of duties, belligerents must treat neutral states’ territory
as inviolable. Belligerents may not move troops, munitions, or supplies, across neutral territory, and they
may not set up communication apparatuses or recruit combatants, among other things, on neutral territory.
Although Hague V and Hague XIII each have fewer than 35 state parties, the United States, Ukraine, and
Russia have ratified both treaties.
Some observers view Hague V and XIII as reflecting customary international law, which is binding on all
countries absent an objection. Others disagree with this view. In 2016, the U.S. Department of Defense
observed that “it may be incorrect to assume” that Hague V and XIII reflect customary international law
when “current events are quite different” from the time the treaties were drafted. Some commentators
have gone so far as to question whether states so frequently ignore neutrality obligations that the treaties
have fallen into a state of obsolescence and are no longer binding. The International Court of Justice has
not directly addressed the customary status of these treaties, but it did state in an advisory opinion that
“the principle of neutrality, whatever its content, … is of a fundamental character” that applies in all
international armed conflicts.
20th-Century Changes: Qualified Neutrality and the U.N. Charter
Some aspects of the law of neutrality have been overtaken by 20th-century developments. Under the U.N.
Charter, for example, the U.N. Security Council can decide upon measures necessary to respond to threats
and acts of aggression. U.N. member states must “accept and carry out” the Security Council’s decisions
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and give the U.N. “every assistance” in its actions. Some Security Council decisions can require member
states to support military operations, impose trade restrictions, and take other actions that could ordinarily
violate neutral states’ obligations. In those cases, the U.N. Charter prevails over neutrality law.
The doctrine of qualified neutrality (also called benevolent neutrality and non-belligerency) also arose in
the 20th century. International law historically contemplated that states could vindicate their rights by
resorting to war in a wide array of circumstances. After World War I, the international system began to
transform following efforts to limit or prohibit war as a method for resolving interstate conflict. This
paradigm shift culminated in Article 2 of the U.N. Charter, which prohibits use of force in most cases and
requires member states to settle disputes by peaceful means. As part of this evolution in international
relations, states and scholars began to contend that a binary system of neutrals and belligerents is no
longer viable, and that modern international law allows for an intermediate position in which countries
can actively assist victims of unlawful wars. Not all states have openly adopted the doctrine of qualified
neutrality, however, and some legal observers argue against its acceptance.
Consequences for Neutrality Violations
Hague V and Hague XIII are largely silent on the remedies available for neutrality violations, but states
and scholars generally agree on certain principles. A violation of the law of neutrality does not, on its own
accord, amount to an “act of war” that creates a legal justification for another state to use force in
response. The U.N. Charter permits states to use force in only three circumstances: approval from the
Security Council, consent of the affected state, or self-defense meeting the standards of Article 51. States
in the modern era, including Russia, use the U.N. Charter’s terminology, not neutrality law, to describe
the legal rationale for their use of force.
A single act could violate both a duty of neutrality and justify the use of force—but only if the act
independently satisfied one of the U.N. Charter’s exceptions. For example, if a state breached its
neutrality obligations by launching an armed attack against a country engaged in an ongoing war, Article
51 could permit the attacked state to use force in self-defense. If a state breached a neutrality duty in a
lesser manner—for example, by failing to detain a belligerent’s ship—the U.N. Charter would not permit
use of force in response.
Breaching a neutrality obligation also does not necessarily terminate a state’s neutral status. Rather, the
state harmed by the breach can choose to continue the neutral relationship, especially if the breach is
“slight and unimportant.” If a belligerent did choose to end the neutral relationship, the belligerent might
also contend that it can take necessary and proportionate countermeasures short of military force.
Co-Belligerency: Becoming a Party to an Existing Conflict
When considering the legal impact of providing military assistance to a state engaged in ongoing armed
conflict, the law of neutrality intersects with other legal frameworks. In particular, the question of when
assistance to a country in a conflict makes the assisting state a party to that conflict—or co-belligerent—
implicates other legal paradigms. The four Geneva Conventions of 1949 and international law governing
state responsibility are relevant to the issue of when military assistance crosses a threshold to co-
belligerency.
The Geneva Conventions provide rules governing the conduct of hostilities in international armed
conflicts (i.e., conflicts between states, such as Russia’s invasion of Ukraine) and non-international armed
conflicts (e.g., conflicts between a state and an organized armed group, such as a separatist group). The
United States is a party to the four Geneva Conventions of 1949 and one of the Convention’s three
additional protocols. The Conventions place certain obligations on each “[p]arty to the conflict,” but they
do not provide great detail on what actions make a state or group a “party” in this context. Some
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commentators, such as the International Committee of the Red Cross, have sought to articulate more
thorough standards for determining when a state’s assistance is sufficiently connected to a belligerent’s
combat operations that the assisting state becomes a party to the conflict. However, no treaty provision or
accepted rule of international law defines the threshold in detail.
The Draft Articles of State Responsibility for Internationally Wrongful Acts provide some rules for
evaluating when conduct can be attributed to a state that supports another group. Some provisions of the
Draft Articles reflect customary international law, but there is still significant debate on their customary
status. Under Article 16, a supporting state can be responsible for violating international law if it “aids or
assists” another state with knowledge of the circumstances of the violation, among other requirements.
Article 8 allows actions to be imputed to a supporting state when the supporting state exercises control of
the persons carrying out the conduct in question—an issue addressed in international tribunals.
The paradigm of state responsibility is premised on the notion that there has been some internationally
wrongful conduct, and its rules address whether those acts can be attributed to a state. In the context of
U.S. military assistance to Ukraine, however, the underlying wrongful conduct could be absent given
Ukraine’s legal right to use force in self-defense under Article 51 of the U.N. Charter. As such, U.S.
military assistance to Ukraine may not neatly fall into the rubric of state responsibility.
Considerations for Congress
Congress may wish to consider neutrality law as it addresses proposals to authorize additional security
assistance to Ukraine, but the law’s antiquated nature and uncertain integration in the U.N. Charter era
make its application less than straightforward. Whether U.S. arms assistance comply with neutrality law
depends in large part on the status of the qualified neutrality doctrine.
The United States used the qualified neutrality doctrine in the World War II era, and thus applying it in
response to Russia’s invasion would not depart from that past legal interpretation. Prior to the United
States’ entry into World War II, then-Attorney General (and later-Supreme Court Justice and chief U.S.
prosecutor at the Nuremburg Tribunals) Robert Jackson articulated the doctrine when defending the
legality of the U.S. program to lend and lease war supplies to the United Kingdom. According to Jackson,
the United States may provide “all the aid we choose” to a government defending an unlawful invasion,
because 20th-century developments “destroyed the historical and juridical foundations of the doctrine of
neutrality conceived as an attitude of absolute impartiality in relation to aggressive wars.” The Ukraine
Democracy Defense Lend-Lease Act of 2022 (S. 3522), which passed in the Senate on April 6, 2022,
adopts the lend-lease model that prompted Jackson’s defense of qualified neutrality. The bill would
authorize the President to lend or lease defense articles to Ukraine and other countries impacted by the
Russian invasion.
Some observers have recommended recalibrating U.S. security assistance to provide more “offensive”
weapons to Ukraine. Neutrality law generally does not use the terminology of “offensive” and
“defensive” military equipment. Hague V and XIII use a broader blanket prohibition on all “war material
of any kind.” Accordingly, U.S. officials’ reference to the defensive nature of its assistance may reflect
more practical considerations, such as the potential for escalation, than constraints in neutrality law.
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Author Information
Stephen P. Mulligan
Legislative Attorney
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