Use of third-state air bases by the United States in the conflict with Iran

The United States has used air bases located in third states in the armed conflict with Iran that began on 28 February 2026. Yet, the action of the United States in Iran may be qualified as aggression, given that neither the conditions of self-defense nor an authorization by the Security Council appear to be satisfied. Proceeding from that premise, this post will analyze the legal position of the third states whose bases the United States used or requested to use. It will show that the justifications provided by third-party states have varied. For example, Portugal, France, and the United Kingdom paid particular attention to the need to avoid assisting the United States in its offensive operations, as they justified the use of their bases as solely for “defensive purposes”. However, the law of neutrality has almost never been mentioned by any State granting or refusing access to the bases. A neutral State would abide by the law of neutrality without furnishing any military assistance. A non-belligerent State would assist without becoming party to the conflict (see also Gill and Tibori-Szabó, p. 334), whereas the assistance of a co-belligerent would be so substantial as to make it a party to the conflict. As in the conflict in Ukraine, States did not refer to this body of law; it is therefore difficult to draw a conclusion concerning their opinio juris about the boundaries between these different statuses. In particular, the issue of co-belligerency remains largely overlooked, even though the consequences for the states in question could be significant, such as the risk of being targeted on their own territory.

This post will classify each third State allowing use of its bases in one of the categories. These cases will mainly be analyzed in the context of the armed conflict between the United States and Iran, which was initiated by the former. The collective self-defense operations of Gulf States, justified by Iran’s unnecessary and/or disproportionate response to the U.S. attack, trigger, in my view, a separate armed conflict: this issue is beyond the scope of this blog post, although reference to it will be made, as States have sometimes justified the use of their bases for this purpose only.

Spain and Italy: compliance with the law of neutrality?

Spain appears to want to remain neutral in this armed conflict. At the outset of the conflict, it not only refused the use of air bases for American operations in the Middle East but also closed its airspace to any overflight connected with those operations. The use by the United States of the Morón and Rota bases nevertheless remains possible under the bilateral defense agreement with Spain. By acting in this way, Spain is complying not only with jus ad bellum, but also with the law of neutrality. Indeed, the Hague Conventions V and XIII of 1907, which reflect customary international law on neutrality, prohibit any military assistance to a belligerent state. Thus, even without declaring its neutrality, Spain is behaving as a neutral state (it is beyond the scope of this blog post whether American soldiers in Spain must be interned.) 

Similarly, Italy refused the use of an air base located in Sicily for American military aircraft transporting weapons for the conflict in Iran. According to Italian officials, this refusal was due to a procedural issue, namely, the United States having failed to submit its request for a necessary authorization in time. In accordance with the applicable bilateral defense agreement, any other use beyond logistical purposes requires parliamentary approval, which could not be granted in time. The justification here was procedural, and therefore not framed in relation to the legality of the American action, although this was mentioned by an Italian political figure: “[…] deny the logistical support offered at all the country’s bases since the US-Israeli conflict in Iran was being carried out ‘in clear violation of international law’.”

Therefore, for Spain and Italy, the restrictions on the use of air bases appear to be consistent with the law of neutrality.

France and Portugal: two examples of states not participating in the conflict but not neutral

These two states appear to occupy an intermediate category. Portugal allowed a military base located in the Azores archipelago to be used by the United States for its military operation in Iran. The Portuguese justification makes no reference to the law of neutrality but instead relies on a 1995 agreement requiring that authorization be requested by the United States. According to the Portuguese government, this authorization must be granted because the use of the base is made “in response to a necessary and proportional strike suffered and which does not target civilians”. This phrase may be interpreted as authorization given to operations carried out in the context of the collective self-defense of the Gulf States. If the use of Lajes base is indeed limited to such actions, then Portugal does not seem responsible for aiding and assisting an internationally wrongful act committed by the United States. However, although it insists that it is not itself a party to any armed conflict, Portugal does not appear to adopt an attitude consistent with the law of neutrality. Consequently, it could be considered a non-belligerent state in relation to the two parallel armed conflicts.

France has a similar position. It allowed the use of the Istres base in the south of its territory, but only for “support” military aircraft, with the assurance that they were not combat aircraft. According to French officials, the posture is defensive and the support is purely logistical (tankers in particular) and not in support of military operations conducted in Iran (aircraft transporting military equipment). Thus, France appears to be in the same position as Portugal: outside the armed conflict between the United States and Iran triggered by the former, but in contradiction with the law of neutrality regarding this conflict. Thus, France seems to adopt the position of a non-belligerent state. The issue of responsibility for aiding and assisting aggression is more delicate: if the refuelled aircraft are later used for offensive operations in Iran, France would have difficulty justifying their stopover at Istres base. If the use serves only the self-defense of Gulf States, then the use of the base would be in compliance with jus ad bellum (and France’s exclusive participation in these defensive operations would also be lawful). This factual point remains uncertain.

Therefore, those countries are not parties to the conflict opposing the US and Iran. However, they do not act as “neutral” in this conflict.

Germany and the United Kingdom: toward aggression or even co-belligerency?

The two most interesting cases are Germany and the United Kingdom, because they tread a fine line between participation and non-participation in the conflict.

For Germany, the use of the Ramstein base is at the center of the issue. Its use is governed by a NATO agreement: the “Status of Forces Agreement (SOFA)” as well as by a “Supplementary Agreement”. Contrary to what one expert argues, the base is not “American soil in Germany”. The states on which such bases are located retain territorial sovereignty over them. In Germany, the controversy is therefore significant: the government maintains that Germany is not a party to the conflict, but the Ramstein base appears to play a very important role in the armed conflict between the United States and Iran. There are two things worth paying attention to. 

The first point concerns the type of aircraft operating from the air base. If combat aircraft take off to participate in offensive operations in Iran, then Germany itself could be considered to have placed its territory at the disposal of the commission of aggression, under Article 3(f) of the annex to UN General Assembly Resolution 3314, which is customary law. Some media outlets noted an increase in C-17 rotations between Germany and the Middle East in the weeks preceding the American attack. Since these aircraft may be used to transport war material, such as tanks, Germany could have committed an act of aggression. At the very least, Germany is responsible for aiding and assisting aggression, under Article 16 of the Articles on State Responsibility for Internationally Wrongful Acts. If, on the contrary, those planes and that material are only used in self-defense of Gulf countries, Germany would not be assisting the US aggression. In any case, compliance with the law of neutrality seems problematic: providing military assistance to a party to an armed conflict is incompatible with neutral status. Germany would also be a non-belligerent State.

The question that may arise, distinct from that of aggression, is that of co-belligerency, because the operations seem to be controlled from German territory. According to a media report, Ramstein base “serves as the control hub for US operations in the Middle East. Data connections and satellite relays for drone operations pass through the facility because direct control from the United States would be too slow ”. Wentker (p. 178 et seq.) has developed criteria for classifying a state a co-belligerent: “direct connection to the harm” and “coordination”. Gill and Tibori-Szabó (p. 342) have stated that “usually — but not always —” co-belligerency materializes once territory is used “as base of operations […] to conduct attacks ”. Wentker’s criteria, drawn from practice, seem convincing. Applied to this case, Germany’s omission to prevent such use of the base does not seem to satisfy the criterion of coordination (Wentker, pp. 186–187): Germany does not appear to be involved in the “decision making process” of American operations. In addition, Germany has repeated that “this is not [its] war”. Co-belligerency therefore does not seem to exist in this case.

The United Kingdom is in a situation where the same questions may arise. Although it had initially refused to grant permission to use its bases, the United Kingdom eventually authorized the limited use of the Diego Garcia base by the United States: it was to be reserved for defensive purposes, in collective self-defense to protect its interests and those of Gulf States, which would concern the separate armed conflict. Use for other purposes would be unlawful. The problem is that the British went further: the use of their base was authorized for military operations aimed at “[degrading] the missile sites and capabilities being used to attack ships in the Strait of Hormuz.” As argued by Marko Milanovic, it is far from certain that Iranian strikes in the Strait of Hormuz against merchant ships can be qualified as armed attacks authorizing the United Kingdom and the United States to use force in self-defense. Thus, the United Kingdom’s authorization for the US to use Diego Garcia’s base would amount to aiding and assisting aggression. But does it itself commit aggression by making the base available? As seen above, Resolution 3314 refers to the making available of “its territory”. It has been argued that, since the Diego Garcia base is on Mauritian territory, this could not amount to the UK committing aggression. The agreement transferring sovereignty over the Chagos archipelago has not, however, to this day, entered into force, and thus Diego Garcia is still under British sovereignty. Accordingly, one may consider that the United Kingdom itself has committed an act of aggression under Article 3(f) of the annex to Resolution 3314 by allowing the US to use Diego Garcia as a military base.

Finally, regarding co-belligerancy, even if it appears that the United States are the only ones carrying out strikes (only “US defensive operations”), the situation regarding the UK differs from that of Germany because the military base in Diego Garcia is under British control, and not exclusively American. Therefore, if one follows Wentker, the criterion of “direct connection to the harm” seems satisfied: the British, by authorizing such actions by the United States, participate in the harm caused in Iran. The criterion of coordination also seems satisfied because the British gave an express authorization to the United States to use the base for that specific purpose and are involved in the “decision-making process”. The United Kingdom could therefore be considered a co-belligerent in the main armed conflict.

Conclusion

Several military bases have been used by the Americans to enable their operations in the Middle East. Depending on how they are used, the position of the third state changes: it either remains neutral, occupies an intermediate position, or becomes a co-belligerent. However, in their justifications, states on whose territory the US operates rarely justify their authorizations using the law of neutrality, but instead seem to engage using the language of jus ad bellum. In any case, it must be concluded that the law of neutrality remains unclear regarding the various limits on the status a state may adopt in relation to a given armed conflict. It triggers questions for further research: is non-belligerency a lawful attitude? What is the exact limit with co-belligerency? Can some operations launched from a third State qualify the use of its territory as its own act of aggression without bringing co-belligerent status? The practice described here, when considered alongside the conflict between Russia and Ukraine in particular, appears to point toward the acceptance of a status that lies somewhere between belligerence and neutrality. The contours of these various statuses remain indeterminate, given the difficulty in ascertaining States’ views on them.