Rolling out a foreign policy that he said would always put America first, Republican front-runner Donald Trump has vowed that if elected president, he would make US allies in Europe and Asia take on more of the financial burden for their defence, or they would be left to defend themselves.
In a major speech on Wednesday, Trump delivered a withering critique of Barack Obama's foreign policy, saying the Democratic president has let China take advantage of the United States and has failed to defeat Islamic State militants.
He pledged to "shake the rust off America's foreign policy" and said he would seek better relations with China and Russia.
The New York billionaire spoke the day after victories in five Northeastern states that moved him closer to capturing the Republican Party presidential nomination for the November 8 election.
Trump, who was also critical of policies of the last Republican US president, George W. Bush, said he would use America's strength sparingly.
He said he would build up the US military to keep pace with Chinese and Russian military programmes but would use American armed forces only when absolutely necessary.
"I will not hesitate to deploy military force when there is no alternative. But if America fights, it must fight to win. I will never send our finest into battle unless necessary - and will only do so if we have a plan for victory," Trump said.
With US-Russian relations strained over numerous issues, including Moscow's support for Syrian President Bashar al-Assad, Trump said "an easing of tensions with Russia from a position of strength" is possible.
Trump, a real estate magnate, also said he would use US economic leverage to persuade China to rein in North Korea's nuclear programme.
"China respects strength and by letting them take advantage of us economically we have lost all their respect," he said.
Trump said he would call separate summits of NATO and Asian allies to discuss a "rebalancing" of the US financial commitment to their defence.
He was stern in charging that American allies have benefited from a US defence umbrella but have not paid their fair share.
"The countries we defend must pay for the cost of this defense. If not, the U.S. must be prepared to let these countries defend themselves. We have no choice," Trump said.
Trump, a reality TV star, has never held elected office and has built support - particularly among white working class voters - with a no-nonsense style and populist pledges to "make America great again."
That message was echoed in his foreign policy speech, although he set aside his rancorous campaign rhetoric for the address on Wednesday, delivered at a downtown Washington hotel.
Trump usually speaks in an off-the-cuff manner, but he delivered the speech with the aid of a teleprompter as he sought to make himself appealing to more Republican voters.
Trump said he would develop a plan to halt the spread and reach of Islamic State militants but that more would be needed beyond the use of military force.
"Events may require the use of military force. But it's also a philosophical struggle, like our long struggle in the Cold War," he said.
Cruz announces running mate
Republican presidential candidate Ted Cruz on Wednesday announced that former business executive Carly Fiorina will be his vice presidential running mate if he wins his party's nomination, media reports said.
Fiorina (61) endorsed Cruz, a 45-year-old US senator from Texas, for the nomination in March, one month after she dropped out of the Republican race herself.
The unusually early announcement of a running mate appeared to be a bid by Cruz to recover from Tuesday's crushing losses to Trump, who swept party nominating contests in five US Northeastern states.
The victories brought Trump closer to the 1237 Republican National Convention delegates he needs to win the nomination at the July 18-21 event in Cleveland.
In a statement, Trump was scornful of Cruz's decision to pick a running mate, calling it "a pure waste of time" and "a desperate attempt to save a failing campaign by an all-talk, no-action politician."
