(AP) -- Donald Trump strained to lay out a clear vision of his "America first" foreign policy on Wednesday, vowing to substitute hard-headed realism for what he called a post-Cold War era replete with U.S. national security failures.

Yet the Republican presidential front-runner outlined no strategy for how he'd make the United States at the same time a "consistent" and "unpredictable" force in the world. And he didn't appear to allay the concerns of U.S. and international critics -- Republicans at home as well as Democrats -- who see him as unfit for the responsibilities of commander in chief.

"'America first' will be the major and overriding theme of my administration," Trump declared, echoing motifs from his campaign that is now close to sealing the GOP nomination.

Delivering his lines in a more sober, restrained manner than usual to cast himself as presidential material, the billionaire businessman and reality TV star toned down or omitted several of his most explosive -- and oft-repeated -- barbs. He made no reference to forcing Mexico to pay for a wall across the nation's Southern border. He also didn't revisit the idea of allowing Japan, South Korea, Saudi Arabia or any other country to develop nuclear weapons for self-defense purposes.

Instead, Trump took aim at President Barack Obama's embrace of an Iran nuclear agreement, Obama's failure to eliminate the Islamic State group and other examples of what he termed a foreign policy of "randomness" and "chaos." His criticism extended to Republicans, too, assailing President George W. Bush for the Iraq war and mocking a foreign policy establishment of "old people" who have scoffed at his diplomatic and national security credentials.

"It's time to shake the rust off America's foreign policy," Trump told academics, reporters and a smattering of supporters in a Washington hotel ballroom after being introduced by one such old voice of the foreign policy elite -- Zalmay Khalilzad, a Bush-appointed former ambassador to Afghanistan, Iraq and the United Nations.

"My foreign policy will always put the interests of the American people and American security above all else," he said. "It has to be first. Has to be. That will be the foundation of every single decision that I will make."

Trump's "new direction" in American foreign policy was vague even by the standards of presidential campaigns.

The U.S. approach to fighting IS will be "strong" under a Trump administration, he vowed, offering no indication of troop levels he might authorize. He intends to ease tensions with Russia while negotiating "from a position of strength," but he wouldn't say what that means for U.S. sanctions punishing Moscow for annexing Ukrainian territory. He said he'd balance America's massive commercial deficit with China "quickly," without a word about tariff policy or existing trade agreements.

Trump also contradicted his own broad-brush approach to foreign policy. He said at one point that the best way to achieve his goals was through "disciplined, deliberate and consistent" policy. Later, however, he exhorted the nation to "be more unpredictable" in how it combats enemies.

"We have to be unpredictable starting now," Trump declared in the 38-minute speech hosted by the Center for the National Interest, an organization founded by President Richard Nixon.

Democratic and Republican opponents immediately pounced on the speech's substance, or lack thereof.

"No one wants an unpredictable ally," said James Stavridis, the retired Navy admiral who served as NATO's top commander in Europe from 2009 to 2013. "It sounds like isolationism."

The "America First" anthem "is precisely what the world worries about," said Christopher Hill, a Bush-appointed former ambassador to Iraq.

But Republican former presidential candidate Newt Gingrich voiced support.

"This was a serious foreign policy speech by Trump," he tweeted. "It is worth reading and thinking about. It will be ridiculed by Washington elites."

Fresh off a sweep of five Northeastern primaries and on a clearer path to nomination, Trump repeated assertions that U.S. allies must contribute more to international security agreements, such as NATO, to reap the benefits of American military protection. Yet he tried to assure allies the U.S. would recommit to them if he's elected president.

Some of the messaging was no different than Obama's, Bush's or any other recent U.S. leader. "We should seek common ground based on mutual interests," Trump said of Russia and China, two leading geopolitical rivals.

But his criticism of the last 15 years of U.S. policy in the Mideast was particularly pointed, coming after weeks when Obama himself has lamented the poor post-war planning in Libya. Trump's likely opponent in the general election, Democrat Hillary Clinton, was Obama's secretary of state at the time and a key architect of the effort.

"We've made the Middle East more unstable and chaotic than ever before," said Trump, lumping in Obama's abandonment of U.S. ally Hosni Mubarak in Egypt, a record of recriminations with Israeli Prime Minister Benjamin Netanyahu and a refusal to enforce his own "red line" for military action after Syrian President Bashar Assad unleashed chemical weapons.

Even before Trump's address, Clinton's campaign blasted his use of "the most reckless rhetoric of any major presidential candidate in modern history."

Some prominent Republican national security voices, too, expressed dismay.

"This is worse than Obama," said Sen. Lindsey Graham, R-S.C., an early loser in the presidential primary season.

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Associated Press writers Lolita C. Baldor and Sagar Meghani contributed to this report.