NATO leaders have agreed to activate a new missile defense shield in three years—openly defying Moscow, which claims the system is aimed at Russian targets.

The transatlantic leaders agreed Sunday to make a sophisticated system operational by 2015 in order to "provide real protection for parts of NATO Europe against ballistic missile attack," Ivo Daalder, U.S. ambassador to NATO, told reporters.

Officials expect to make the system initially capable in 2015, before declaring it fully capable three years later.

The envisioned system has helped contribute to increasingly icy relations between Washington and Moscow. U.S. officials say the system is designed to intercept Iranian missiles aimed at both European targets and U.S. interests in the region. Moscow believes it could instead take down Russian missiles should they be fired.

Washington has secured agreements from Poland, Romania, and Turkey to place elements of the envisioned shield on their soil. Daalder announced Spain also has agreed to host part of the system.

The alliance sent a monetary message directly to Russian leaders, agreeing to devote $1 billion to support the Moscow-opposed shield.

"This decision ... now means that NATO has, for the first time," Daalder said, "a territorial missile defense capability."

The missile agreement comes after Obama administration officials sounded a defiant tone toward Moscow in the days before the summit.

"There is nothing I can imagine that will keep us from deploying the system as planned," Ellen Tauscher, a U.S. special envoy for strategic stability and missile defense, told reporters earlier this month.

Barry Pavel, a former White House and Pentagon official, says NATO officials had little choice but to move forward with plans for the system.

"The key question is this: 'Is there really any chance for progress on this issue with the Russians?' " Pavel says. "The Russians know this system is not aimed at them. If it was, we'd have interceptors in northern Europe, not southern Europe. This thing would be in Greenland, Estonia and England."

"The Russians know this," Pavel says. "The Russians simply want to use this as a political tool."

Russian officials so far have been silent about the agreement.

John T. Bennett covers national security and foreign policy for U.S. News & World Report. You can contact him at jbennett@usnews.com or follow him on Twitter.