WASHINGTON, United States (AFP) – Republican White House hopeful Donald Trump promised yesterday a top-to-bottom overhaul of US foreign policy to put “America First” and make its allies pay up.

In a speech short on specifics that left Washington policy experts scratching their heads, Trump warned that Europe and Asia may have to defend themselves.

He vowed to tear up trade deals, retool NATO to oppose migration and “radical Islam”, and put US national interests ahead of all other considerations.

“’America First’ will be the major and overriding theme of my administration,” he said, co-opting the slogan of America’s pre-World War II isolationists.

The speech had been billed to give Trump, the clear frontrunner for the Republican nomination, a chance to win over a sceptical foreign policy establishment.

But many were quick to point out the contradictions in a strategy of restoring US strength and crushing the Islamic State group while cutting allies loose.

“Our foreign policy is a complete disaster,” he said, dismissing all presidents to have served since the Cold War, both Republicans and Democrats.

“We’re rebuilding other countries while weakening our own,” he said, decrying nation-building missions in the Middle East and the US trade deficit with China.

“Our allies are not paying their fair share,” he said, pointing the finger at both NATO and close Asian allies such as Japan and South Korea.

“The countries we are defending must pay for the cost of this defence. And if not, the US must be prepared to let these countries defend themselves.

“We have no choice,” he warned, complaining of the “trillions of dollars” he claimed have been wasted shoring up Cold War-era defences.

Despite this, he quickly pivoted to an attack on President Barack Obama, accusing him of abandoning US allies like Israel while reaching out to Iran.

“We picked fights with our oldest friends and now they’re starting to look elsewhere for help. Remember that. Not good,” Trump declared.

He was clear, however, on where the greatest threat to the United States now lies, accusing Democratic frontrunner Hillary Clinton of a refusal to name it.

“Containing the spread of radical Islam must be a major foreign policy goal of the United States and, indeed, the world,” he said.

David Pollock, a former senior State Department advisor and now Washington Institute fellow, said the speech would help Trump with Washington experts.

“It seemed to be a more appealing presentation and Trump themes in a softer and smarter way, but left many questions unanswered,” he told

AFP.

“So people are probably kind of taking a sceptical stance, but then see what he follows up with,” he said.

“And a key test of that will be what kind of new advisors he can find who will publicly side with him and join the team.”

Trump has been widely mocked for failing to secure big-name foreign policy endorsements and relying on a coterie of right-wing fringe figures.

But he defended his team, slamming the available talent in Washington circles as tainted by the chaos unleashed under President George W Bush.

“My goal is to establish a foreign policy that will endure for several generations,” he said.

“That’s why I also look and have to look for talented experts with approaches and practical ideas rather than surrounding myself with those who have perfect resumes but very little to brag about except responsibility for a long history of failed policies and continued losses at war.”