In 2018, Mr Trump berated his allies over military spending and claimed that Nato "benefits Europe far more than the United States". As a result, Nato states agreed to increase their defence spending by $100 billion in January last year.
Earlier this month, Mr Trump withdrew the US from the Open Skies Treaty, an international agreement that allows unarmed surveillance flights over member countries.
Donald Trump has ordered the Pentagon to slash the number of troops it maintains in Germany by more than a quarter in the coming months, according to a report.
The move would significantly reduce the United States' commitment to European defences under Nato and comes amid tensions between the Trump administration and European allies over longstanding co-operation agreements. Mr Trump has criticised Germany for not spending enough on defence.
The Wall Street Journal said the Defense Department would cut the number of military personnel by 9,500 from the 34,500 troops permanently posted in Germany.
The newspaper also said a cap of 25,000 would be set on how many US soldiers could be inside Germany at any one time, including temporary postings, which would be half of the current allowance.
White House and Pentagon officials declined to comment but a spokesman for the White House National Security Council said: "The United States remains committed to working with our strong ally Germany to ensure our mutual defense, as well as on many other important issues."