A bipartisan coalition of lawmakers is pressing President Obama to provide weapons so that Ukrainians can defend themselves against Russian aggression.  As the New York Times noted June 11: “The Senate has included provisions in its military policy bill to arm Ukraine with anti-armor systems, mortars, grenade launchers and ammunition to aid in its fight against Russian-backed separatists. It would also prevent the administration from spending more than one half of $300 million in aid for Ukraine unless 20 percent is earmarked for offensive weapons. The House has passed a similar measure.”

At the same time there is growing pressure from our NATO allies in the Baltic States and Eastern Europe, in particular the states that border Ukraine, for more U.S. military personnel and systems to help defend them against the growing threat of Russian aggression.

Obama, however, remains opposed to helping the Ukrainians fight back and to permanently stationing U.S. combat troops in the Baltic states or Eastern Europe. So what does he offer instead? A new plan to preposition a brigade’s worth of tanks, armored personnel carriers, and other heavy equipment in various Eastern European states. According to the Times, “As the proposal stands now, a company’s worth of equipment — enough for about 150 soldiers — would be stored in each of the three Baltic nations: Lithuania, Latvia and Estonia. Enough for a company or possibly a battalion — about 750 soldiers — would be located in Poland, Romania, Bulgaria and possibly Hungary.”

Prepositioning U.S. military equipment isn’t a bad idea. In an exercise or crisis, it allows troops to fall in more quickly on the equipment—much faster than lugging tanks, etc., from the United States. But it’s no substitute for a presence of U.S. military presence.

The chief problem the U.S. faces vis-à-vis Russia (and every other bad actor on the planet) is a lack of credibility. Putin simply doesn’t believe we are going to make a substantial commitment to stop him—any more than Ayatollah Khamenei or Abu Bakr al-Baghdadi imagine we’re gong to stop them. The only way to restore American deterrence and credibility is by putting U.S. troops on the frontlines, as we did in Germany during the Cold War and as we are still doing in South Korea. You don’t have to deploy a lot of American troops—certainly not enough by themselves to stop the Russian Army or even the North Korean army. Just enough to serve as a tripwire and delaying force, ensuring that any aggression will put American personnel in danger and thus require an American military response. Empty tanks and APCs aren’t the same—no one imagines that we would go to war to protect them.

And that’s why the Obama plan will fall far short of its objective, of deterring Russian aggression. Doing that effectively requires arming the Ukrainians and stationing U.S. brigade combat teams in Eastern Europe and the Baltics.

Some—including the president himself—would no doubt argue that this would raise the risk of war with Russia. But what really raises the risk is uncertainty about American responses to Russian aggression. Putin may be tempted to send his “little green men” to invade, say, Estonia, because he doesn’t believe the U.S. and our other NATO allies would meet their Article V obligations under NATO. Putin may well find that he is mistaken—that just as the U.S. responded to the North Korean invasion of South Korean in 1950, so too we might well respond to Russian attacks today. But rather than risk a confrontation based on uncertain levels of commitment, it’s better to signal up front that we will defend our allies no matter what. We learned during the Cold War how American military deployments could keep the peace. Alas, that seems to be a lesson we’ve forgotten or rather choose to ignore. Stalin wouldn’t have been deterred by empty U.S. tanks from seizing West Berlin—and for that matter, all of West Germany–and neither will Putin be deterred from seizing whatever real estate he has his eye on next.