evidence: Before the seizure of Crimea, both Ukraine and Russia agreed to regard the Sea of Azov as the internal waters of each state. Thus, the states had shared sovereignty over that body of water; in the case of disputes, they would resolve them jointly. [...] However, the issue of the Sea of Azov does not end here: there is more at stake than just Ukraine. The international community cannot recognize the right of Russia to seize territory by force, annex it, and then pretend that it has always been and will continue to be Russian. That opens the door to further adventures across the former Soviet and even Tsarist empire, jeopardizing every state from Finland to Bulgaria. [...] Clarifying to Moscow and everyone else that the United States will defend freedom of navigation and Ukrainian sovereignty in the Black Sea and the Sea of Azov is no less important than doing so in the South China Sea. And it is high time that we did so along with our NATO allies; this is as vital an interest to them as it is to us.