The US and Russia are facing off in a potentially escalating military confrontation. Any incident that could provoke a clash between the world’s two largest nuclear powers carries serious risks. And that risk is growing as the two countries have departed from longstanding nuclear arms control treaties and moved to develop new kinds of weaponry.

The latest clash between the US and Russia was triggered by a Russian warplane’s close encounter with an American drone in international airspace over the Black Sea. A video released by the Pentagon shows the Su-27 making two passes and dropping what appears to be fuel over the drone. The US official who made the release said the drone’s rear propeller was damaged during the encounter.

Russian Foreign Minister Sergey Lavrov complained that the drone had violated Russia’s declaration of temporary airspace restrictions over the Crimea peninsula, which Moscow seized and illegally annexed in 2014. He also said that the US had been trying to harass the planes. The Pentagon says the drone strayed into the war zone and that its remote pilot wiped sensitive information from the aircraft before it crashed in the sea.

US-Russia relations have sunk to their lowest level since the Cold War, and more Americans now view Russia as an adversary than as an ally or a friend. The deterioration has accelerated over the past year as the Obama administration has sought to check Putin’s aggression in Ukraine and catalyze a revived and more unified NATO.