President Trump must respond to the aggressive action by a Russian Udaloy-class destroyer in the Philippine Sea on Friday. U.S. Navy video proves the Russians maneuvered towards the USS Chancellorsville in an unsafe manner. The Russians will attempt to flood the Navy's YouTube page with troll comments, but the video speaks for itself.

The Russians clearly found their activity hilarious: In another video recorded by the U.S. Navy, Russian sailors are shown sunbathing on their helicopter deck just prior to the closing incident. Regardless, Trump cannot let this stand.

For a start, it sits alongside increasingly regular incidents of similar kind. The overriding Russian intent here is intimidation: Moscow wants to foster an American understanding of strategic challenge and vulnerability. But it is also notable that the USS Chancellorsville incident occurred in the Philippine Sea and follows a summit between Vladimir Putin and Chinese President Xi Jinping. The Philippine Sea is the epicenter of China's effort to dominate the West Pacific and subjugate the U.S.-led international order under Beijing. This Russian harassment thus allows Putin to show Xi that he can be a powerful strategic ally. Putin wants Chinese investment.

Trump must act here. As with China, where the U.S. military rightly ignores Chinese demands that it suspend operations in international waters, Trump should make clear to Putin that his escalation will meet American resolve. Trump should double down on recent U.S. military activities that overmatch the Russians. The president should also order more regular U.S. Navy deployments into the Black Sea. This will upset the Russians in a strategic manner.

Yet Trump should also make clear that the Chinese-Russian relationship, and Russian aggression as in this video, are threats to international order, threats to a system that has made nations richer and people more free. This is how the U.S. can win great democracies such as India and Pacific nations such as Vietnam to our cause.

Let China and Russia be friends, and let us be friends with everyone else.