When Is The Use Of Military Force Permitted In International Law?
Members of the United Nations have an "inherent right of individual or collective self defence if an armed attack occurs against a member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security." U.N. Charter Article 51. And, "the practice of those States which can do so has been to invoke the right of self-defence to protect their nationals and shipping and certainly to protect their armed forces." (Source).
But in the absence of an armed attack or U.N. Security Council authorization, defense against "the threat or use of force against the territorial integrity or political independence of any State" (U.N. Charter Article 2(4)) is prohibited, and no other use of force against a member state is authorized by international law.
Application To The Drone Incident
The tactic of a fuel dump which from the evidence looks like it was intentional, was likely used to muddy the question of whether there was an armed attack by a Russian military warplane on a U.S. military drone, whose affiliation with Russia and the U.S., respectively, was not in doubt, in international water and airspace, was really an "armed attack" within the meaning of international law.
But given the context and the close proximity of the Russian warplane to the American military drone, it is hardly a stretch to call it an armed attack, particularly if one considers the warplane and not just the fuel it used as "ammunition" in the attack as what made the attack "armed."
The bottom line is than multiple repeated intentional acts of aggression from a Russian warplane downed a U.S. military drone.
Therefore, this attack on an American military drone by a Russian warplane was a violation of international law, for which the U.S. would be justified in retaliating in defense of its military.
Russia could, however, argue that the American military drone, while technically in international airspace, was being used to aid Ukraine in waging war against Russia's military forces by providing reconnaissance and targeting information to Ukrainian military forces (which the U.S. has previously admitted to doing, at least in what were clearly authorized leaks if not also in officially authorized statements or confirmations). Ukraine uses information from drones like this one to attack Russia. Indeed, Ukraine often uses this kind of information to attack Russian military forces with U.S. supplied military weapons like guided artillery rockets and guided howitzer shells.
In that case, Russia could argue that its strike on the American drone was justified by its right to self-defense (to the extent that this is not forfeited by its status as an aggressor due to its illegal invasion of Ukraine and continued war since 2014 with Ukraine on territory that Russia previously agreed in treaties belongs to Ukraine).
Was The Drone Attack An Act Of War?
Lesser provocations that are less well documented have been deemed acts of war. The U.S. would be justified in declaring that Russia had declared war upon it in this incident.
But, the characterization of an act of another nation for purposes of international law as an act of war is something that is for the subject of the attack to decide subjectively and in light of its diplomatic agenda. It is not an objective question with a clear answer apart from the desired of the diplomatic high officials and heads of state in each country. Ultimately, characterizing this act is rude behavior by the Russian military, as the U.S. has done in many other recent run ins with Russian aircraft in the last couple of years or so, or as an act of war, is a decision for President Biden with the advice but not consent of his diplomatic, political, and Congressional advisors, to make.
One argument that either side could make in support of the position that this was an armed attack, but was not an act of war, is that the pilot of the Russian warplane was going rogue and acting without authorization or sanction from the Russian government in attacking the U.S. military drone. This can't really be ruled out from the publicly available evidence including the video evidence.
The U.S. military is likely to have signals intelligence (i.e. intercepted communications) from the Russian warplane that would clarify whether this was a rogue pilot's actions or an attack ordered by Putin personally, or something in between. But, the U.S. military and the President may decide that this information is best left classified, at this time, to give the U.S. maximum diplomatic and political flexibility to deal with this incident.
A conclusion that the drone incident was the act of a rogue Russian fighter pilot would justify U.S. self-defense against the attack, although now that the attack is over that justification may have faded away. But it would prevent the drone incident from constituting an act of war, because it is only an act of war if the aggrieved party (i.e. the U.S.) attributes an intent to make war on it to the armed attack made by the aggressor (i.e. Russia).
If the U.S. determines that this incident was the an act of a rogue Russian fighter pilot, and Russia was content to deny was an act of war against the U.S., what consequences follow from this determination?
If the need for further self-defense of U.S. forces has been abated, then the most that the U.S. might reasonably demand from Russia as a result of this incident is that the pilot be disciplined by Russia, or turned over to a U.S. military tribunal, and that the U.S. be compensated for the cost of the drone.
What If It Isn't An Armed Attack?
If this is not an armed attack, it might be a violation of international aviation law, but that generally pertains only to civilian aircraft, not to encounters between military aircraft near a war zone. Normally the remedy for an accidental or civil violation of civil regulations would be a lawsuit in the courts of Russia, the U.S., or an agreed third-party tribunal, or diplomatic demands for economic compensation for the harm done perhaps with some fine attached.
Application To The Russian Invasion Of Ukraine
Separately and additionally, because the Russian invasion of Ukraine was a a clear violation of U.N. Charter 2(4), and Ukraine has a right of collective self-defense against such acts of aggression against the territorial integrity and political independence of Ukraine (which Russia had previously affirmed voluntarily by treaty), the U.S. would be justified under the U.N. Charter in any necessary military action to repeal that Russian invasion of Ukraine in support of Ukraine's territorial integrity and political independence.
But, the consequences of this violation of international law by Russia is up to the military and diplomatic resources of the countries in question. There is no settled tribunal for the mandatory legal resolution of this violation of international law.
Accusations of violations of international law merely structure diplomatic discussions about whom other countries should side with in international military conflicts, but each nation must judge the law, the evidence, and what it should to about its conclusion drawn from these points itself, without any binding third-party guidance.
The U.N. has never had the authority or the means of enforcing its charter's constraints on the lawful use of military force between U.N. Security Council members, something that has been an issue with regard to the effectiveness of the U.N. at preventing wars since its inception.
Russia can and would veto any U.N. Security Council action, so this cannot be a justification for the use of force against Russia either.
Thus, while Russia is clearly in the wrong in terms of international law, the only remedy available is righteous individual and collective self-help by the United States and its allies to defend themselves against Russia. Ultimately, this incident didn't do much to advance the cause of action against Russia since good cause for it already existed.
Relevance Beyond International Law
This latest downing of a U.S. drone in breach of international law is mostly relevant because it may provide U.S. politicians with justification in their discretion to take more aggressive action against Russia now that the U.S. has been directly the subject of an armed attack by Russia.
But, the U.S. is also under no obligation to do so and the fact that the U.S. and Russia are the world's leading nuclear armed nations bodes caution before escalating a run-in that did not cost a single American life into a nuclear World War III.