Donald Trump vows in foreign policy speech to put 'America first'

In a rare policy speech, Republican front-runner Donald Trump vowed Wednesday to put American security "above all else" if elected president, warning allies they would be left to defend themselves if they don't "pay their fair share."

"'America first' will be the major and overriding theme of my administration," Trump said.

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Fresh off a sweep of five Northeast primaries, Trump sought to expand on foreign policy views that have lacked detail and worried experts in both parties. He panned U.S. President Barack Obama's handling of crises in the Middle East, saying the current administration was leaving a legacy of "weakness, confusion and disarray."

"We've made the Middle East more unstable and chaotic than ever before," Trump said. He singled out Democratic front-runner Hillary Clinton's handling of the deadly attacks on a U.S. compound in Benghazi, Libya.

Much of the speech was similar to his typical campaign remarks, but it was delivered in a much more sober, restrained manner. Critics have said repeatedly that he has not shown an ability to act and sound "presidential."

Trump spoke to an invited audience of conservative-leaning national security experts, as well as some prominent foreign policy writers.

He read his speech off a teleprompter, a notable change for a candidate who has mocked his rivals for doing the same and typically speaks off the cuff. He has declared: "If you're running for president you shouldn't be allowed to use a teleprompter."

Trump's advisers cast Wednesday's speech as the first in a series of policy addresses aimed at calming the nerves of Americans who worry the businessman doesn't have the experience to handle the range of responsibilities that land on a president's desk.