The U.S. government should improve Ukraine's ability to defend itself against Russian aggression. While U.S.-led sanctions pressure has an important role to play here, that alone won't protect Ukraine.

An experienced strategic analyst privately observed to me that weapons are key here. American weapons can structurally improve Ukraine's military and alter the balance of power in Kiev's favor.

Two weapons programs stand out. First, the U.S. should sell Ukraine the PAC-2 Patriot air defense system. PAC-2 has long been on Kiev's wishlist, in that it would give Ukraine's military the means of better defending its forces against Russian invasion. Russian military doctrine suggests that its forces would seek to rapidly isolate Ukrainian military bases and strong points in any open conflict. But if Russia suffers significant losses to its air power component, its ability and inclination to advance will be restrained.

In addition, the U.S. should provide Ukraine with at least two mothballed U.S. warships of the Oliver Hazard Perry class. Tyler Rogoway wrote about Ukraine's interest in these vessels last month. The U.S. will not be going to fight over the contested Sea of Azov anytime soon, so it is up to Ukraine to defend its waters. While it would take some months to get these decommissioned U.S. warships effectively integrated into the Ukrainian navy, their provision by Washington would rebuke Moscow and help Kiev. But as with the PAC-2 Patriot missile sales, the main opportunity of these ships would be in deterring further Russian aggression.

To be sure, Moscow will act outraged by both these actions. That's just too bad. President Vladimir Putin has wantonly ignored the diplomatic process that he claims to support, so there are few options short of active war fighting. The U.S. doesn't want a war, of course, but it should not become a neutral actor in this struggle.