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(CNN) --
President Joe Biden has approved sending 3,000 US troops to Poland, Germany and Romania to reinforce NATO countries in Eastern Europe, a deployment abroad that is nothing unusual.
The deployments, US officials have told CNN, are a show of support for NATO allies who feel threatened by the buildup of Russian troops near the Ukrainian border.
The United States announces the deployment of thousands of soldiers in Eastern Europe in the face of the crisis in Ukraine
The White House and the Pentagon have stressed that the US military will not enter Ukraine.
"These are moves designed to respond to the current security environment," Pentagon press secretary John Kirby said on Wednesday, adding that these forces "are not going to fight in Ukraine."
While the troop reshuffling is the most significant sign yet that the United States is preparing for the prospect of a Russian invasion of Ukraine, the movement of US soldiers abroad is not at all unusual.
And there are a number of options under which a president can deploy troops.
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Here's what you need to know about it:
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Sending US soldiers to exercises and bases abroad
The United States can and often does send soldiers to Europe to participate in exercises with partner nations.
American soldiers are deployed to bases across the continent, and these exercises and stations are based on agreements with each nation individually or with a treaty alliance like NATO.
Bilateral agreements or requests to send soldiers
These deployments are made in consultation with allied nations and with the agreement of both countries.
And this is the dynamic that is playing out now, as America's NATO allies have called in American soldiers to bolster their own defenses and send a stronger message of deterrence to Russian President Vladimir Putin.
But while NATO allies are receiving US troops, this is a move that is a US deployment made in consultation with these allies, not NATO's own deployment.
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Operations abroad in the framework of NATO
As part of the North Atlantic Treaty Organization, the US has also committed to assisting allied countries and providing NATO's overall defense.
Part of that defense involves NATO's Response Force, a 40,000-strong multinational force.
In 2014, partly in response to the Russian invasion of Crimea, NATO created the Very High Readiness Joint Task Force, which is part of the Readiness Action Plan to provide rapid responses to a changing security environment.
NATO has not activated the NATO Response Force - which has also been used in disaster relief - in regards to Russia's actions on the Ukraine border, but the Pentagon has said it has 8,500 soldiers on high availability just in case.
And the United States and NATO have tens of thousands of other soldiers already in Europe to call on for any additional deployments to Eastern European allies.
Kirby said Wednesday that the troops being deployed are separate from the 8,500 US troops on high alert.
The Pentagon "doesn't rule out the possibility of more" US troop movements abroad in the coming days, Kirby said.
In 2005, NATO Response Force aircraft delivered NATO relief supplies to the United States in the aftermath of Hurricane Katrina, and last August the NATO Response Force was activated to support the evacuation of Afghans fleeing they had previously worked with the alliance.
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Authorized use of military force by the United States
This is the most controversial option on the list, but one that has been used repeatedly to send soldiers abroad since the attacks of September 11, 2001. So far, no authorization has been sought from Congress for the use of force in relation to with the tensions between Russia and Ukraine.
The 2001 Authorization for the Use of Military Force comprises only one long sentence.
But his 60 words have helped pave the way for years of military activity.
According to official documents, Presidents George W. Bush, Barack Obama and Donald Trump invoked it during their mandates.
Although both Republicans and Democrats have expressed a desire to update the 2001 Authorization for the Use of Military Force, which authorized the global war on terror, and the 2002 authorization for the war in Iraq, the consensus has bogged down amid disputes over how a new authorization would address the scope of the mission, time constraints and the use of US ground troops.
The Constitution, for its part, explicitly states that only Congress has the authority to declare war, a power that has been invoked five times in United States history: The War of 1812, the Mexican-American War, the Spanish-American War. America and the First and Second World Wars.
With information from Natasha Bertrand, Barbara Starr, and Jeremy Herb.
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