US Election 2016: Trump laments 'ripped off' America
US presidential hopeful Donald Trump has vowed to pursue an "America First" foreign policy, saying many nations, including allies, "ripped off" the US.
The frontrunner for the Republican nomination for the November election was speaking to the New York Times.
Mr Trump said he would consider pulling out troops from Japan and South Korea if they did not pay the US more.
He said he might stop buying oil from Saudi Arabia if it did not send troops to fight so-called Islamic State.
Mr Trump insisted he was "not isolationist" but "America First".
He said: "We have been disrespected, mocked, and ripped off for many, many years by people that were smarter, shrewder, tougher.
"We were the big bully, but we were not smartly led. And we were the big bully who was the big, stupid bully and we were systematically ripped off by everybody."
The thrust of Mr Trump's interview concerned the economic cost of US foreign policy.
Mr Trump cited the US debt - "soon to be $21tn" (£14.8tn) - and linked it to the fact the US "defended the world".
"No matter who it is, we defend everybody. When in doubt, come to the United States. We'll defend you. In some cases free of charge."
He said that China had rebuilt itself from money that has "drained out of the United States".
"They've done it through monetary manipulation, by devaluations. And very sophisticated. I mean, they're grand chess players at devaluation."
He added: "I like China very much, I like Chinese people. I respect the Chinese leaders, but you know China's been taking advantage of us for many, many years and we can't allow it to go on."
Mr Trump also cited Japan, South Korea, and nations in the Middle East as not paying their way.
Mr Trump said the US was "not being properly reimbursed" for protecting Saudi Arabia.
"Without us, Saudi Arabia wouldn't exist for very long. It would be, you know, a catastrophic failure without our protection.
"They're a money machine... and yet they don't reimburse us the way we should be reimbursed."
He also cited Nato, saying it was "obsolete" as the main international threat now was terrorism.
"Nato is something that at the time was excellent. Today, it has to be changed."
The US, he added, bore "far too much of the cost of Nato".
Mr Trump expressed a similar view about the US funding of the United Nations.
He said: "We get nothing out of the United Nations. They don't respect us, they don't do what we want, and yet we fund them disproportionately again."
Mr Trump also referred to the criticism he had received for calling Brussels a "hellhole waiting to explode" but said that, after the deadly attacks on Tuesday, he had been proved right.
He spoke of the arrest before the attacks of key suspect Salah Abdeslam.
Mr Trump, who has said he supports the use of some methods of torture in some cases, said that if Belgian authorities had "immediately subjected him to very serious interrogation - very, very serious - you might have stopped the bombing".
Mr Trump also touched on Mexico, having previously accused Mexican immigrants in the United States of being criminals and rapists, and vowing to build a wall on the border.
He said US jobs were being lost to Mexico "and it has to be stopped".
Many Mexicans have been outraged by Mr Trump's stance. On Saturday, an effigy of him was burned in Mexico City on Saturday in a twist on the Easter "Burning of Judas" tradition.