Republican front-runner Donald Trump has vowed to seek better relations with Russia and China if elected president and says he would make America's allies bear more of the financial burden for their defence.
Mr Trump, the frontrunner for the Republican candidacy in the 2016 presidential race, said he would pursue an "America First" policy.
He called the foreign policy of President Barack Obama's administration "a complete and total disaster" and pledged to "shake the rust off America's foreign policy".
On Tuesday, Mr Trump called himself the Republican "presumptive nominee" after his primary wins.
He claimed victories in Connecticut, Delaware, Rhode Island, Maryland and Pennsylvania.
Before the speech, he promised it would not be a "Trump doctrine", and that he would retain some flexibility to make changes if elected.
Mr Trump vowed that if elected president, he would make US allies in Europe and Asia take on more of the financial burden for their defense, or they would be left to defend themselves.
He also delivered a withering critique of Mr 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 seek better relations with China and Russia.
Mr Trump, who was also critical of policies of the last Republican U.S. 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 programs 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," Mr Trump said.
With US-Russian relations strained over numerous matters including Moscow's support for Syrian President Bashar al-Assad, Mr Trump said "an easing of tensions with Russia from a position of strength" was possible.
Mr Trump, a real estate magnate and reality TV star, also said he would use US economic leverage to persuade China to rein in North Korea's nuclear program.
"China respects strength and by letting them take advantage of us economically we have lost all their respect," he said.
Mr Trump said he would call separate summits of NATO and Asian allies to discuss a "rebalancing" of the US financial commitment to their defense.
He was stern in charging that American allies have benefited from a US defense umbrella but have not paid their fair share.
"The countries we defend must pay for the cost of this defense. If not, the US must be prepared to let these countries defend themselves. We have no choice," Mr Trump said.
Mr Trump 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 IS 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.
Mr Trump said he would also aim to hold talks with Russia to try and establish common ground, possibly over Islamic extremism.
"Some say the Russians can't be reasonable," he said. "I intend to find out."
China, he said, "respects strength, and by letting them take advantage of us economically like they are doing, we are losing all their respect". He said he would seek to "fix our relations with China" but did not suggest how.
After his sweep of the five mid-Atlantic states, Mr Trump said of the battle for the Republican nomination: "It's over. As far as I'm concerned, it's over."
He told supporters in New York he would not moderate his policies if elected president.