The US is set to withdraw almost 12,000 troops from Germany in what it described as a “strategic” repositioning of its forces in Europe.
About 6,400 troops will be sent home, with the rest moved to other Nato countries such as Italy and Belgium.
President Donald Trump said the move was a response to Germany failing to meet Nato targets on defence spending.
But it has attracted widespread opposition in Congress from those who believe it will embolden Russia.
Senior German officials have also expressed concern.
“We don’t want to be the suckers anymore,” Mr Trump told reporters at the White House on Wednesday shortly after the move was announced. “We’re reducing the force because they’re not paying their bills; it’s very simple.”
Mr Trump has long complained that European members of Nato should spend more on their own defence.
He has said that Nato members should no longer be relying so heavily on the US to shoulder the costs of maintaining the alliance.