In response to the Russian invasion of Ukraine, NATO Secretary General Jens Stoltenberg today announced activation of the alliance's defense plans, which means also that it can deploy the NATO Defense Force. The U.S. Defense Department now stands ready to provide capabilities to help fill that defense force, if asked to do so, said Pentagon Press Secretary John F. Kirby.

"There's a historic nature to all this," Kirby told reporters during a briefing at the Pentagon today. "This is the first time that the alliance has employed these high readiness forces in a deterrence and defense role. So it's not an insignificant move by the alliance."

The activation serves as a notice to NATO nations that they may be called upon to provide military support to the NATO mission, Kirby said. This also means the United States may be called upon to provide both personnel and equipment to support the NATO mission — and that's something the U.S. and the department has been preparing to do for quite some time.

Kirby said what kinds of assistance NATO may request from the United States, or other alliance nations, is unknown at this time, as is what specific U.S. military units may end up being assigned to what roles.

Still, Kirby said, the U.S. has put as many as 12,000 service members on "prepare to deploy orders" so they will be ready if called upon to participate in the NATO Response Force. Some of those U.S. personnel may also be called upon to participate in any unilateral actions the U.S. may undertake.

"The department has placed a range of multi-mission units in the United States and Europe on a heightened preparedness to deploy, which increases our readiness to provide for the U.S. contributions to the NRF on a shorter tether than what we could do before," Kirby said. "We stand ready if called upon by NATO to support the NRF in the defense of the alliance, and will absolutely do that."

While President Biden has said U.S. troops will not enter Ukraine to participate directly in the fight against Russia, U.S. forces may eventually be called upon to participate in NATO-led missions as part of the NATO Response Force to bolster the defense of alliance partner nations should they ask for assistance.

"We're going to do everything that we need to do to defend our country, and as the President has said, we're going to do what we need to do to defend every inch of NATO territory — and we take those obligations seriously," Kirby said.

At the same time, Kirby said, the U.S. continues to provide security assistance to Ukraine to help that country defend itself against an unlawful invasion by Russia.

"We're continuing to look for ways to support Ukraine to defend themselves," Kirby said. "We have continued to do that and we're going to look to do that going forward. And we're very actively engaged in those efforts, to help them better defend themselves through both lethal and non-lethal assistance."