Ceasefire in International Armed Conflict: Implications for Jus Ad Bellum Self-Defense

In recent months, various commentators have proposed that Ukraine should agree to a ceasefire with Russia as a means of ending the ongoing hostilities of the Russo-Ukrainian War. This raises an underexplored issue under the laws of war: how does a ceasefire agreement impact the jus ad bellum rights of a State engaged in self-defense during an ongoing international armed conflict?

The Concept of a Ceasefire and its Effects

A ceasefire is an agreement that effectuates the suspension of ongoing hostilities between opposing belligerents. While a somewhat amorphous concept, the “suspension of hostilities” is most accurately defined as “a suspension of active military operations of a hostile character.”

Because it does not conclusively terminate hostilities, an indefinite, general ceasefire—one which has an undefined duration and applies to all hostile interactions between the opposing belligerents—does not end the armed conflict between the parties. However, it is unclear how a ceasefire impacts the authority of the opposing parties to subsequently conduct hostilities against each other.

One prominent view, articulated by Professor Yoram Dinstein, is that because the armed conflict continues, an indefinite, general ceasefire does not ultimately place any limits on the parties’ right to resume hostilities. According to this perspective, parties to an indefinite ceasefire may resume hostilities “at will” upon notice to the opposing belligerent (Dinstein, War, Aggression, and Self-Defense,  p. 59-61). This position finds support in Article 36 of the Hague Regulations, which provides that the parties to an agreement that merely suspends hostilities between them may “resume operations at any time.”

However, other scholars question the continued validity of this rule under the modern jus ad bellum. For example, according to Professor Christopher Greenwood, given the centrality of the use of force prohibition to the contemporary jus ad bellum, a general ceasefire of indefinite duration must be interpreted as a mechanism for transitioning to the conclusive termination of hostilities (Handbook of International Humanitarian Law,  p. 67-68). As a result, while such an agreement does not end an armed conflict, it does result in the reapplication of the use of force prohibition to the relations between the opposing belligerents. Accordingly, any resumption of hostilities is lawful only if it complies with jus ad bellum self-defense requirements.

This perspective is supported by the Friendly Relations Declaration, which prohibits “the use of force to violate international lines of demarcation, such as armistice lines, established by or pursuant to an international agreement.” As Tom Ruys and Felipe Silvestre argue, this duty can be interpreted as prohibiting the use of armed force in violation of contact lines established under a ceasefire agreement.

Both of those scholarly views, however, are problematic on logical and doctrinal grounds.

Assessing Competing Doctrines

The traditional perspective is deficient because it cites authorities which presume the ceasefire includes terms that establish the circumstances in which hostilities may be resumed. Article 36 of the Hague Regulations provides that parties to a ceasefire may “resume operations at any time, provided always that the enemy is warned within the time agreed upon, in accordance with the terms of the armistice” (emphasis added). That language suggests that a party to an indefinite ceasefire may resume hostilities at will only if that agreement contains terms for providing notification to the opposing belligerent.

Contemporary ceasefire agreements, however, tend not to explicitly address the issue of how hostilities may be resumed. Absent such provisions, it is doubtful that Article 36 applies to those agreements. Moreover, the notion that the parties to an indefinite ceasefire can resume hostilities at will when that agreement does not expressly provide for that contingency is logically inconsistent with the principle that States must perform their international obligations in good faith. As the U.S. Department of Defense (DoD) Law of War Manual makes clear, that norm requires parties to a ceasefire to faithfully adhere to their obligations under the agreement (§ 12.13).

Critically, the principal duty imposed by any ceasefire is a prohibition on conducting “offensive military operations” against the opposing party. Absent any specific temporal duration or procedures for recommencing hostilities, good faith compliance with such an agreement logically precludes either party from recommencing hostilities “at will.”

However, while this means that an indefinite ceasefire establishes a legal obligation on the parties to refrain from resuming hostilities against each other, it does not mean that the parties’ right to resume hostilities is governed by the peacetime jus ad bellum. As discussed, it is generally accepted that a ceasefire does not end an armed conflict. That reality is inconsistent with the conclusion that an indefinite ceasefire brings the full protections of the peacetime jus ad bellum back into operation as between the opposing belligerents. Instead, the peacetime prohibition on the use of force only resumes when the opposing parties are no longer engaged in armed conflict.

Accordingly, the better view of the effects of a ceasefire on the jus ad bellum relationship between the opposing parties is that the terms of the ceasefire—not the peacetime rules on self-defense—govern the parties’ authority to use force against each other (Heintschel von Heinegg, Factors in War to Peace Transitions, p. 848-49). Importantly, this means that the jus in bello rules governing ceasefire agreements will further inform the circumstances under which the opposing parties may use force against each other.

Under those rules, the prohibition on conducting active hostilities established under a general ceasefire is conditioned upon the opposing party’s compliance with that agreement. As articulated in Article 40 of the Hague Regulations, the parties to a ceasefire may denounce the agreement and resume hostilities if the opposing party commits a “serious violation” of its terms. Critically, those rules distinguish the jus ad bellum situation established by a ceasefire from that which emerges upon the conclusion of an armed conflict, because a serious ceasefire violation may involve conduct that does not give rise to the right of self-defense under the peacetime jus ad bellum.

Because a ceasefire effectuates an absolute prohibition against active hostilities, acts of force falling short of an armed attack could be viewed as a serious ceasefire violation. Moreover, a ceasefire can impose additional obligations on the opposing parties beyond the prohibition against conducting offensive military operations (DoD, Law of War Manual, § 12.12.2). For example, a ceasefire might prohibit the belligerents from carrying out reinforcing military deployments. Arguably, a violation of such terms could constitute a “serious breach” of a ceasefire.

A limited, yet insightful, body of State practice supports the foregoing conclusions.

Ceasefires in State Practice

In 1967, large-scale hostilities between Israel and an Arab coalition led by Egypt and Syria ended with an indefinite, general ceasefire that was agreed to between the opposing parties upon a request—not a demand—from the UN Security Council (UNSC). Subsequently, in 1973, Egypt and Syria reinitiated hostilities against Israel, initially claiming that their renewed offensive had been launched in response to ongoing Israeli violations of the 1967 ceasefire (The Use of Force in International Law: A Case-Based Approach, p. 192–93). When that claim was proven to be inaccurate, Egypt and Syria nonetheless argued that prior Israeli violations of the 1967 ceasefire had terminated that agreement and therefore afforded them the right to resume hostilities against Israel in the course of an ongoing armed conflict.

For its part, Israel claimed that the resumption of hostilities by Egypt and Syria constituted “a massive violation of international law” because it breached the 1967 ceasefire. According to Israel, the ceasefire was “an international agreement” that obligated the opposing parties to abstain from hostilities against each other and respect the lines of contact established under that instrument. Because the ceasefire was generally stable when Egypt and Syria launched their offensive, those renewed hostilities constituted “aggression” by those States.

Critically, during the 1973 hostilities, several influential third States also assessed the legal position of the active belligerents in reference to their obligations under the 1967 ceasefire, rather than the peacetime jus ad bellum. The United States, for example, noted that the Arab offensive had breached the ceasefire and called for its restoration. It did not, however, claim that the Arab powers had engaged in an illegal use of force and explicitly rejected efforts to assign responsibility to either side for unlawfully initiating the overall armed conflict.

More recent—albeit controversial—practice further supports the above conclusions. In 2002 to 2003, for example, the United States and the United Kingdom claimed they possessed the authority to use force against Iraq because Iraq was in “material breach” of UNSCR 687. In that resolution, which suspended the hostilities of the 1991 Gulf War, Iraq agreed to a ceasefire with the U.S.-led coalition that had been authorized to use force against it in UNSCR 678. Among other things, the ceasefire required Iraq to destroy its weapons of mass destruction and consent to a UN inspections regime that would verify that disarmament process. In UNSCR 1441, the UNSC had assessed that Iraq was in material breach of that ceasefire due to its failure to comply with those obligations.

According to the U.S.-UK position, by identifying Iraq’s material breach of the ceasefire, UNSCR 1441 “revived” the authority to use force against Iraq contained in UNSCR 678 that had been suspended by UNSCR 687. That view was broadly criticized by other States. As articulated in a 2002 Office of Legal Counsel opinion, however, ceasefires established in UNSC resolutions may be interpreted in accordance with general armistice law principles (DoD, Law of War Manual § 12.14; see also Michael Schmitt, The Legality of Operation Iraqi Freedom Under International Law, p. 379).

Consistent with that reasoning, in 1992, the UN Legal Counsel explained that the 1991 ceasefire obligated the U.S.-led coalition to cease hostilities against Iraq under certain conditions. “As long as those conditions pertained,” the coalition’s authority to use force against Iraq was “suspended.” However, “a sufficiently serious violation of Iraq’s obligations” under the ceasefire could “withdraw the basis for the ceasefire” and therefore permit the resumption of hostilities against Iraq.

That reasoning was used by members of the U.S.-led coalition to justify the renewed use of force against Iraq in 1993 and 1998. Importantly, in those cases, hostilities were resumed against Iraq in response to Iraqi ceasefire violations that did not amount to a use of force, including Iraqi failures to comply with its disarmament obligations. The true controversy in the 2002-03 “revival debate,” therefore, was not whether a serious Iraqi ceasefire violation could permit the resumption of hostilities against Iraq. Instead, it was the issue of which actor had the authority to determine that a serious violation had occurred and that hostilities should be resumed in response.

As the UN Legal Counsel explained, since the 1991 ceasefire was agreed to between Iraq and the U.S.-led coalition acting under the auspices of the UNSC, the UNSC—and not individual coalition members—needed to determine that an Iraqi ceasefire violation “was such that all means . . . were justified in order to bring Iraq back into compliance” with the agreement.

Therefore, the claim that new UNSC authorization to use force against Iraq was required in 2003 was consistent with the reasoning that a resumption of hostilities could be justified based on general ceasefire principles.

Conclusion

The foregoing indicates that an indefinite, general ceasefire suspends the parties’ authority to conduct active hostilities against each other, subject to the opposing belligerent’s compliance with the agreement. A serious violation of the ceasefire by one party—including through conduct that does not constitute an armed attack—permits the opposing belligerent to denounce the agreement and resume hostilities.

Accordingly, if a State engaged in self-defense during an ongoing international armed conflict agrees to an indefinite, general ceasefire, it indefinitely suspends its jus ad bellum authority to employ armed force against the aggressor for as long as the aggressor complies with the agreement. The prospect of a Russo-Ukrainian ceasefire therefore raises the question whether Ukraine—and third States supporting Kyiv’s war effort—are prepared to accept this conditional limitation on Ukraine’s war of self-defense against Russian aggression.

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Photo credit: Sgt. Jason Greaves