ARLINGTON, Va. — On his visit to Ukraine this week, the American defense secretary, Jim Mattis, confirmed that he favors providing “defensive weapons” to the former Soviet republic. According to recent reports, this military aid would involve sending Javelin anti-tank guided missiles to Ukraine, which has been fighting Russian-backed separatists in the eastern Donbass region for more than three years.

The Trump administration’s plan to arm Kiev is a serious political decision that could have far-reaching strategic consequences. The United States is walking into a proxy war with Moscow — one that it is unprepared to win.

Notionally, helping Ukraine is admirable, but the lack of public discussion and suddenness of this announcement are worrisome. While arming Kiev may seem like an easy political win, it is poor policy. The idea of providing Ukraine with $50 million-worth of anti-tank missiles is eerily reminiscent of Washington’s halfhearted efforts to train and arm the moderate Syrian opposition.

That plan was ill-conceived and ended in defeat after Russia escalated its military backing of the government of President Bashar al-Assad in 2015. Russia’s interests in Ukraine are far greater and its military superiority is well established. In contrast, the United States’ coercive credibility in the region is close to nonexistent.