His latest tweetstorm comes a day after he met German Chancellor Angela Merkel in Washington, where the two leaders showed little common ground over a host of thorny issues, including NATO and defense spending.
"Germany owes vast sums of money to NATO & the United States must be paid more for the powerful, and very expensive, defense it provides to Germany!" Trump tweeted on Saturday morning.
He prefaced his statement by lashing out at the news media. "Despite what you have heard from the FAKE NEWS," he tweeted, "I had a GREAT meeting with German Chancellor Angela Merkel."
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That appeared to be far from the case on Friday, when the veteran German leader arrived hoping to reverse a chill in relations after Trump had said during his campaign last year that her decision to allow refugees into Germany was a "catastrophic mistake" and suggested she was "ruining Germany."
But during a joint news conference, Trump accused Germany of unfair trade practices and ripped into Washington's NATO allies, demanding they pay back "vast sums of money from past years."
Merkel said Germany had committed to increasing its military spending to two per cent of GDP, a target NATO member states formally agreed in 2014 to reach within 10 years.
A German government spokesman declined to comment about Trump's tweets on Saturday, referring AFP to Merkel's statements on the subject during Friday's news conference.
Treaty commitment
Trump had made European defense spending an issue during his campaign, saying the United States - which spends just over three percent of its GDP on defense - carries too much of the financial burden for supporting NATO.
However, critics pointed out on Saturday that NATO members don't pay the United States for security, but contribute by spending on their own militaries.
"Sorry, Mr President, that's not how NATO works," tweeted Ivo Daalder, a former US ambassador to NATO. "This is not a financial transaction, where NATO countries pay the US to defend them. It is part of our treaty commitment."
"We fought two world wars in Europe, and one cold war," he added. "Keeping Europe whole, free, and at peace, is vital US interest."
US defense spending - $679 billion in 2016 - accounts for nearly 70 per cent of the total defense budgets of NATO's 28 members.
But member states resolved to increase their defense spending after the dramatic events of 2014, when Russia annexed the Black Sea peninsula Crimea from Ukraine and began backing separatist rebels in eastern Ukraine.
Then, the Islamic State group declared a "caliphate" just across NATO's southern border in Syria and Iraq.
NATO members agreed on a 10-year plan to each increase their national defense spending to two percent of their respective GDPs.
Five - Britain, Estonia, Greece, Poland and the United States - have met that goal. Three more - Latvia, Lithuania and Romania - are expected to do so this year.
Last year, according to the alliance, 23 of the 28 member states increased their defense spending in real terms, the first time that has happened in more than two decades.
"This is not a business ledger sheet with credits and debits," another former US ambassador to NATO, Douglas Lute, told AFP. "It's a 10-year investment program and allies are making progress, slowly."
Trump has also worried US allies by criticizing the military alliance as "obsolete" and failing to meet the challenge posed by Islamic terror groups.
Added value
Germany, whose militaristic past has led it traditionally to be reticent on defense matters, currently spends 1.2 per cent of GDP.
But the country's defense minister has called for changes to the way NATO members' commitments to budget targets are assessed.
Speaking on Friday ahead of Merkel's trip to Washington, Ursula von der Leyen told AFP that the two percent target paints an incomplete picture of actual contributions, saying member states that take part in NATO operations and exercises or contribute personnel and hardware should get credit toward the two per cent goal.
"For me, the question is who is really providing added value to the alliance," she said.
Von der Leyen proposed using an "activity index" that would take participation in foreign missions into account when assessing budget earmarks for defense.
