“This doesn't seem to be completely spelled out in detail,” Foreign Minister Frank-Walter Steinmeier said of Trump's declaration.
The Republican frontrunner had attempted to dispel fears that he had little foreign policy expertise with a much-hyped speech in Washington.
He promised to “always put the interest of the American people and American security above all else” - so far, so uncontroversial.
But he repeated his insistence that Nato allies including Germany were “not paying their fair share” of defence costs.
"The countries we are defending must pay for the cost of this defense, and if not, the U.S. must be prepared to let these countries defend themselves," Trump said.
He also promised to intimidate Russia and China with shows of strength, and suggested using military force against the Isis militant group.
The bombastic businessman said that he believed nuclear weapons were the biggest threat to the world while at the same time insisting that the USA must upgrade its own arsenal.
“America is going to be strong again,” he vowed.
From a German point of view, The Donald appeared to be proposing a smorgasboard of policy priorities and actions that didn't all line up.
Making America strong while withdrawing from many of its commitments abroad “don't seem to me to fit together too well,” Steinmeier said.
“The world's security architecture has changed and it is no longer based on two pillars alone,” Steinmeier said, referring to the Cold War polarization between US- and Soviet-led blocs.
“No American President can get around this change... 'America first' is no answer to that.”
"I can only hope that the election campaign in the USA does not lack the perception of reality," Steinmeier added.
It's not the first time Steinmeier has taken a thinly-veiled swipe at Trump as the scion of privilege has inched closer to the Republican nomination for November's Presidential vote.
In March, the Foreign Minister used the opportunity of a visit to the USA to reject “politics of fear” and insist that “building walls is a very bad idea – no matter who pays for them” - a clear reference to Trump's plan to build a wall along the US border with Mexico.