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In western Poland, U.S. troops and tanks are participating in military drills. They're part of a force of 4,000 American servicemen and women who were deployed to Eastern Europe earlier this month. It's the biggest U.S. military buildup in the region since the Cold War and Russia has spoken out against it. It sees the exercises as a threat to its borders.

It's not just U.S. troops who are involved. It's other members of NATO. And though the American forces were initially sent by former President Barack Obama, it appears their work there has the support of President Donald Trump. That's significant because he's repeatedly questioned the U.S. role in NATO.

According to the U.S. Army's lead commander in Europe, though, President Trump has reassured several other NATO members that the U.S. remains committed to the organization.

NIC ROBERTSON, CNN SENIOR DIPLOMATIC EDITOR: What is NATO? Why is it important? And what's its future?

The North Atlantic Treaty Organization is a political and military alliance established in 1949 that seeks to promote stability in the North Atlantic area.

UNIDENTIFIED MALE: It is a will of the people of the world for our freedom and for our peace.

ROBINSON: Led by Secretary General Jens Stoltenberg, there are 28 member countries and its HQ is in Brussels.

NATO doesn't have his own troops but relies on contributions of forces from its member countries. At NATO's core is Article 5 which states an attack on one member is an attack on all NATO allies.

The collective defense principle was to protect Western European nations against the Soviet Union. But when the Soviet Union collapsed, NATO's new tasks range from being a bulwark against al-Qaeda in Afghanistan to fighting human trafficking and intercepting refugees in the Mediterranean.

Donald Trump has called it obsolete, says it has problems.

DONALD TRUMP, PRESIDENT OF THE UNITED STATES: NATO is obsolete.

ROBERTSON: He thinks NATO's original purpose of protecting against the Soviet Union makes it irrelevant, that it's ill-equipped to tackle terrorism and that other members don't pay their share towards it.

NATO is still extremely active with some 4,000 U.S. troops in Poland and the Baltic States, and tens of thousands on 48-hour standby — bolstering NATO's allies and sending a clear message to Russia.

But the organization's future and its principle of collective defense could be jeopardized if Trump pulls the U.S. out. Some of his cabinet picks back NATO, though, like General James "Mad Dog" Mattis, chosen to head the Pentagon, have categorically declared NATO necessary to the USA.