President Obama announced Tuesday that he is seeking up to $1 billion to boost the military presence in Eastern Europe, a move meant to assure European allies that the United States is willing to stand up to any further eastward encroachment from Russia.

The package must be approved by Congress. It would support an upsurge in military personnel dispatched to Eastern Europe, support allies with training and add additional U.S. Navy resources to a NATO deployment in the Black and Baltic seas.

The United States has long chided NATO allies for scrimping on defense spending. But Obama's announcement in Poland underscores that the United States is NATO's largest funder, and the alliance would not have its military might -- and effectiveness -- without U.S. backing.

"We have no choice but to take the lead," said Michael E. O'Hanlon, a senior fellow at the Brookings Institution. "The alliance is best viewed as a glass half full. The reason it’s not all the way full is the allies don’t do anything close to their fair share."

In March, Obama said he was concerned that not all NATO members were "chipping in" toward the cost of a collective defense against Russia. In 2011, then Defense Secretary Robert Gates made a stinging rebuke of NATO allies, saying in Berlin that the United States is tired of engaging in combat with and taking risks for countries that “don’t want to share the risks and the costs.”

Last month, NATO announced plans to substantially increase its air, sea and ground presence in the Baltic states in response to Russia's intervention in Ukraine, which left some of NATO's newest members in the region fearing a possible threat to their borders. 

That announcement came after a meeting attended by, among others, Defense Secretary Chuck Hagel. Smaller nations closer to Russia have signaled that they are willing to spend more to defend themselves.

According to NATO's 2013 annual report, the United States was responsible for 73 percent of the alliance's spending on defense, up from 68 percent in 2007. The United Kingdom contributed 6.6 percent of the defense spending, France 4.9 percent and Germany 4.7 percent in 2013.

According to our colleague Ernesto Londoño, NATO  leaders demanded that members spend at least 2 percent of their GDP on defense, but many have not met that benchmark. Italy spent 1.7 percent of its GDP on defense in 2012, Poland 1.9 percent, Spain 0.9 percent and the United States 4.2 percent.

O'Hanlon said so far the alliance is working "pretty darn well" on European security, and it is striking how the entire alliance is committed to protecting the territorial integrity of each member country. O'Hanlon said he believes that there is now "no plausible Russian threat to NATO that I can see" and he is hopeful the situation won't get worse.

O'Hanlon, however, said that even though the alliance is thriving he can't think of a time when the United States was pleased with how much other countries were contributing to defense.

"It’s sort of a perennial thing. I can’t think of any high-level American official who has ever been happy with the European contribution to collective defense," O'Hanlon said. "To some extent it’s a reflex. When you look at the numbers it’s an inevitable conclusion you have to come to."

And as Obama reminded his allies, defense costs money.

"The situation in Ukraine reminds us that our freedom isn’t free, and we’ve got to be willing to pay for the assets, the personnel, the training to make sure we have a credible NATO force and an effective deterrent force,” Obama said at the Council of the European Union in Brussels in March.