When it comes to U.S. security interests in Europe, the Pentagon must balance military needs with the need for NATO allies to do more.

That concern is again relevant following Senate testimony on Tuesday in which America's top military officer in Europe called for more military assets to be delegated to his command. General Curtis Scaparrotti, who is also NATO's supreme commander, says he needs more intelligence assets and two more destroyers, to add to the four already present. And Scaparrotti was clear with the senators."I am not comfortable yet with the deterrent posture that we have in Europe,” he said.

The basic issue here is not that Scaparrotti needs two more U.S. ships, but that the U.S. needs its NATO allies to free up their own ships. The central problem is that NATO members continue to limit the general's ability to deploy their destroyers where he needs to. Put simply, into the Black Sea. Only the U.S. and Britain are willing to do so with regularity.

The Black Sea is key to the deterrent posture against Russia in that Vladimir Putin's sustained aggression in Ukraine represents a strategic testing ground for how NATO will respond to his escalation. That's why the U.S. is rightly increasing its Black Sea naval deployments. But there's no question that European NATO states could fill the gap.

After the U.S. in NATO, the French navy is second only to Britain's Royal Navy. Continuing with his recent courage in taking a tougher stance against Russia, French President Emmanuel Macron could confidently send a number of different warship classes into the Black Sea. But other states could also do far more. When it comes to air defense frigates, a key concern against Russian capabilities, for example, the Spanish navy's Alvaro de Bazan class and Germany's Sachsen class vessels are highly capable. Italy also has a number of advanced frigates that it has developed alongside the French navy.

There are a lot of NATO naval assets that are highly capable but currently unused. They should be in the Black Sea to deter Russia. The problem is that each of the aforementioned nations is reluctant to contribute. Their preference is to avoid actions that would aggravate Putin. Instead, they choose to leave the hard work of NATO deterrent resolve to the U.S. and Britain.

That must change.

Of course, if the U.S. keeps filling in the deployment gaps, its allies never will. President Trump must demand that European allies step up to the plate. They can and should do so. America cannot keep filing in the gaps.