President Joe Biden believes that the US will expand its military presence in Europe as part of reshaped plans for the continent's security.
For the first time, there will be a permanent US garrison in Poland, creating an enduring American foothold on the alliance's eastern flank.
Mr Biden said the US would send two additional squadrons of F-35 fighter jets to the United Kingdom and more air defences and other capabilities to Germany and Italy.
In Madrid, Biden said that the United States will strengthen our force posture in Europe, respond to the changing security environment and strengthen our collective security.
He said that the US would keep 100,000 troops in Europe for the foreseeable future, up from 80,000 before the war began in Ukraine.
The NATO Secretary-General Jens Stoltenberg, who said the alliance was facing its biggest challenge since World War II because of Russia's invasion of Ukraine, welcomed Mr Biden's announcement.
This shows your decisive leadership and strength in the transatlantic bond, Mr Stoltenberg said.
The US military presence is not quite as large as it was during the Cold War, when approximately 300,000 American troops were stationed in the region, but it signals a renewed focus on European security.
By the end of the year, NATO plans to increase the size of its rapid-reaction force from 40,000 to 300,000 troops.
The troops would be based in their home countries, but they would be ready to deploy further east, where the alliance would stockpile equipment and ammunition.
Mr Biden said that the US would increase its deployments of troops to Romania and the Baltic region, in addition to permanently stationing the US Army V Corps forward command in Poland.
Celeste Wallander, an Assistant US Secretary of Defence for International Affairs, told reporters that having a long-term presence in Poland will be key to helping NATO navigate the changing security environment in Europe.
Polish President Andrzej Duda, who is in Madrid, stated on Twitter that the permanent presence of US military command structure was an extremely important decision and we have been waiting for US officials to emphasise that the permanent bases applied only to headquarters units, not combat troops.
They said it was consistent with the agreement between NATO and Russia in 1997, in which the alliance agreed not to permanently base combat troops in Eastern Europe post-Cold War.
The combat units that Biden is sending to Romania and the Baltic region are deployed on rotational deployments rather than permanent assignment to stay in compliance with that agreement.
After its invasion of Ukraine, NATO branded Russia the biggest threat to Western security and agreed on plans to modernise Kyiv's beleaguered armed forces.
At the Madrid summit, NATO invited Sweden and Finland to join and pledged a seven-fold increase in combat forces on high alert along its eastern flank against any future Russian attack.
In response to President Vladimir Putin's comments, Russia would respond in kind if NATO set up infrastructure in Finland and Sweden after they joined the US-led military alliance.
Russian news agencies quoted Putin as saying he could not rule out that tensions would arise between Moscow's relations with Helsinki and Stockholm over their joining of NATO.
Mr Putin thought NATO members would splinter after he invaded Ukraine, but got the opposite response.
Mr Biden said that Putin was looking for Finland-ization of Europe.
You're going to get the NATO-ization of Europe. That's exactly what he didn't want, but exactly what needs to be done to ensure the security of Europe.