Russia’s attack Sunday on Ukrainian naval vessels in the Azov Sea shows the Kremlin is ready to take the veil off the conflict. After nearly five years of clandestine war using private armies, proxies and unmarked federal forces, the Russians announced themselves loudly with an open strike on Ukrainian forces.

The aggression specifically underscores Russia’s willingness to use naval power to block commerce and restrict trade routes. The attack violated a 2003 treaty that designated the Azov Sea as shared territory between...