Republican front runner Donald Trump has promised an "unpredictable" foreign policy when dealing with America's enemies if he wins the White House.
In a speech designed to burnish his national security credentials, he complained that too often the US telegraphs its plans to foes.
Mr Trump vowed to get rid of the Islamic State group "very, very quickly" if elected, but said he would not provide details so as to catch the jihadists off guard.
"We must as a nation be more unpredictable," he told his audience at a Washington DC hotel. "We are totally predictable.
"We tell everything, we're sending troops, we tell them, we're sending something else, we have a news conference.
"We have to be unpredictable. And we have to be unpredictable, starting now."
Mr Trump also warned allies they would be left to defend themselves if they don't "pay their fair share".
Speaking a day after landslide victories in five primaries , he said it was time to "shake the rust off" US foreign policy.
Mr Trump said America's world standing had been rendered "a complete and total disaster" under the Obama administration, with "no vision, no purpose, no direction, no strategy".
He said Mr Obama was leaving a legacy of "weakness, confusion and disarray", which he argued Democratic presidential front runner Hillary Clinton would only continue.
The billionaire businessman said he would get out of the nation-building business and focus unyieldingly on what is best for the US, in an "America first" approach.
"President Obama has not been a friend to Israel," Mr Trump also said in his speech, which despite an isolationist strain was largely in line with the conservative world view.
"He's treated Iran with tender love and care."
Mr Trump also appealed to Russia and China, saying the US and those world powers were "not bound to be adversaries".
He read the 38-minute remarks off a teleprompter - unusual for a candidate known for his freewheeling style.
Critics have said repeatedly the property magnate lacks an ability to act and sound "presidential".
Mr Trump spoke to an invited audience of conservative-leaning national security and foreign policy experts.