Ian J. Brzezinski, a senior fellow at the Atlantic Council, was deputy assistant secretary of defense for European and NATO policy from 2001 to 2005.

NATO’s response to Russia’s invasion of Ukraine has drawn a red line, but it is one that leaves Ukraine militarily isolated, fending for itself. If the West’s economic and diplomatic sanctions are to deter Moscow from further military aggression, they must be complemented by a robust defensive strategy to reinforce Ukraine’s armed forces.

When Russia invaded Crimea, it mobilized 150,000 troops along Ukraine’s eastern frontier. Most of those forces still menace Ukraine, with some 20,000 troops still occupying the peninsula while provocateurs sent by Moscow continue to stir unrest in the country’s eastern regions.

NATO’s response has, by contrast, been underwhelming. The United States and Britain reinforced the air space of Estonia, Latvia and Lithuania with a handful of fighter jets, and AWACs patrols fly over Poland and Romania. The United States deployed about a dozen F-16s to Poland and sent an additional ship to the Black Sea. No ally appears to have mobilized any ground forces.

When Ukrainian Prime Minster Arseniy Yatsenyuk met with President Obama this month, his request for weapons that would enable his military to better defend against Russia’s massed forces was politely declined. Instead, the Obama administration offered uniforms and military meals.

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In a similarly negative move, Vice President Biden visited Warsaw and Vilnius, Lithuania, last week to reassure them of the U.S. military commitment to their security, but he bypassed Kiev. This was surely noted by Moscow, as was Obama’s recent statement that he would not allow the United States to get involved in a “military excursion” in Ukraine.

These U.S. and alliance actions constitute a red line that depicts Kiev on the outside and on its own. This must be deeply disillusioning for Ukrainians who in recent months have so courageously expressed their desire for freedom and a place in Europe — and whose forces participated in a NATO collective defense exercise as recently as November. This red line can only reassure Vladi­mir Putin and his military planners, whose use of unmarked military personnel — and the plausible deniability they provided — in Crimea reflected at least initial concern about potential responses from the West.

There are prudent defensive measures the United States and NATO can and should take to bolster Ukraine’s security. First, Yatsenyuk’s request for military equipment should be immediately approved, and anti-tank and anti-aircraft weapons should be included. Equipment and weapons could quickly be transferred from prepositioned U.S. military stocks in Europe.

If NATO cannot attain the consensus to initiate such assistance, then Washington should forge a coalition of the willing or act on its own. These weapons would complicate Russian military planning and add risk to its operations against Ukraine. U.S. equipment in particular would bring back unpleasant memories of when Soviet forces encountered Western weapons in Afghanistan.

Second, the alliance or a U.S.-led coalition should back that assistance with the deployment of intelligence and surveillance capabilities and military trainers to Ukraine. This would provide not only needed situational awareness and help the Ukrainian military maximize its defensive capacities, but it would also force Moscow to consider the potential political and military repercussions of any actions that affect that presence. The deployment of military trainers to Georgia was one of the more effective elements of the U.S. effort to bolster Georgia’s security after it was invaded by Russia in 2008.

Third, NATO allies and partners should soon conduct a military exercise in Ukraine as part of the effort to train the Ukrainian military. The alliance’s plan to wait until its next scheduled exercise in Ukraine, this summer, could incentivize Russia to take additional military action before then.

The NATO Response Force, created to deploy on short notice a brigade-level force backed by combat air support, is well suited for such an exercise. The force offers a means to demonstrate Western resolve prudently and rapidly. It has the potential to significantly reinforce Ukraine’s defense against a sudden Russian offensive, but it is not big enough to jeopardize Russia’s territorial integrity.

Each of these initiatives would complicate Putin’s ambitions regarding Ukraine and could be executed in the near term. None would present a threat to Russia. They would, however, amend the red line the alliance has mistakenly created, assure Ukrainians that they are not alone and force Moscow to consider the possibility of a much more costly and prolonged military conflict. The absence of a firm Western response will only encourage Putin to act aggressively again, be it to drive deeper into Ukraine, make another attempt to seize Georgia, expand Russia’s occupation of Moldovan territory or grab other areas that were once part of the Soviet Union.

NATO’s response to this crisis is critical to both Ukraine’s security and the alliance’s long-term future. A NATO summit planned for September is to focus on the alliance’s way forward in a new world. But what it does to assist Ukraine today and in the coming weeks will have a far more profound influence on its future and transatlantic security.