US President Joe Biden has vowed that he would defend every inch of NATO territory. He addressed members of the House Democratic Caucus on Friday.

Mr. Biden said that  US has placed 12,000 troops along the borders with Russia, such as Estonia, Lithuania, Latvia and Romania and stated that Putin will not emerge victorious in this war.
He said he talked to Ukraine President Zelenskyy for an hour “The people of Ukraine have demonstrated a remarkable source of bravery and courage.  But the security assistance that you’ve provided has been absolutely critical — critical in their defence,” he said. 
“When Putin unleashed his assault, he thought he could divide NATO.  He thought he could literally divide this country, the West — the United States — in terms of the parties.  I know a former ambassador knows that out there.  He thought he could divide Democrats and Republicans at home.  But he failed.  He failed,” said Biden
“For months, the United States has worked tirelessly to keep our Allies and partners together.  As our former ambassador — who you see on television a lot, thank God — will tell you I’ve spent literally hours and hours and hours and hours keeping the Alliance together — the EU, NATO, and all of our allies, including our Asian allies.”
“As a result, we’ve been able to ramp up our economic pressure on Putin and further isolate Russia on the global stage,” he said. 
"That's why I've sent 12,000 American troops along Russia's borders - Latvia, Estonia, Lithuania, Romania, and so on... To be sure, if we respond, it will be World War Three. But we have a holy duty on NATO territory... notwithstanding the fact that we will not fight a third World War in Ukraine."