WASHINGTON (AP) — In his final stretch as president, Barack Obama is driving the United States toward friendlier relations with longstanding adversaries, working to consign bitter enmities with Vietnam, Iran, Cuba and Myanmar to the history books.

Though the reconciliations have been years in the making, Obama hopes he can prove the benefits of his softer approach before he hands control to an uncertain successor in January. Defiant cries of naiveté by his opponents have only strengthened his conviction that the U.S. must release itself from an us-versus-them mentality forged during wars that ended decades ago.

The quest for resolution was on display this week in Hanoi, where Obama lifted an arms sales embargo that had stood as one of the last remnants of the Vietnam War and the deep freeze that persisted until the two nations restored relations in 1995.

Obama's next gesture will come Friday in Hiroshima, Japan, where he'll become the first sitting president to visit the site where the U.S. dropped the first atomic bomb — helping end World War II but sowing resentments. Seven decades later, those have mostly fallen away. Though his move has rankled some U.S. veterans and some Japanese, Obama's visit will be a powerful reminder of the intimate alliance between two nations that now view China more warily than they do each other.

Speaking to the Vietnamese people Tuesday, Obama dismissed calls for keeping the communist-run country at a distance, the stance of those fecklessly nursing long-forgotten rivalries. He noted that he's the first president to come of age after the war, telling his young audience that his own daughters had grown up knowing only peace between the U.S. and Vietnam.

"When the last U.S. forces left Vietnam, I was just 13 years old," Obama said. "So I come here mindful of the past, mindful of our difficult history, but focused on the future: the prosperity, security and human dignity that we can advance together."

For Obama, the belief that his youth uniquely positions him to turn the page took root long before he was elected president. In his 2006 book "The Audacity of Hope," Obama wrote that American politics suffered from a case of arrested development, or what he dismissively referred to as "the psychodrama of the baby boom generation — a tale rooted in old grudges and revenge plots hatched on a handful of college campuses long ago."

Elsewhere, in recent years few countries have seen as dramatic a shift in U.S. relations as Myanmar, also known as Burma. With the country's transition away from decades of oppressive military control, the administration rewarded Myanmar for reforms by easing sanctions against state-run companies and banks earlier this year while continuing to call for more economic and political changes.

Though the party of longtime opposition leader Aung San Suu Kyi swept into power in Myanmar in March, Suu Kyi herself is still barred from formally holding the presidency due to a constitutional rule believed to have been written specifically for her. Ben Rhodes, Obama's deputy national security adviser and an architect of his Myanmar policy, said the rapprochement reflected dual goals of acknowledging history but not becoming imprisoned by it.

"That's kind of central to the president's whole view of the world," Rhodes said. "That we can move beyond difficult and complicated histories and find these areas of common interest."

The sprint toward warmer relations with erstwhile foes reflects Obama's hope that by locking in tangible progress, he can make his approach seem inevitable and even irreversible. While likely Democratic presidential nominee Hillary Clinton largely supports Obama's detente with Cuba, Iran and Myanmar, Republican Donald Trump is a wild card whose election could augur a sharp return to the more belligerent U.S. stance of the past.

Yet Obama's critics argue that in his efforts to make peace, Obama has erroneously lumped together countries like Myanmar and Vietnam, which have gradually moved toward U.S. values, with others like Iran and Cuba, which they say have not.

Sen. Tom Cotton, R-Ark., one of the harshest critics of Obama's foreign policy, says the outreach to Cuba and Iran demonstrates "the folly of the president's rush to appease adversaries who don't change their behavior."

"The United States should always be open to trying to turn adversaries or former adversaries into allies, but it has to be based on their conduct," Cotton said in an interview.

The president's diplomatic successes in a handful of countries are tempered by backsliding elsewhere, such as in Iraq and Syria, where Obama concedes the U.S. will be fighting the Islamic State group long after he's out of office. In Russia, where Obama held out early hope for a new era of cooperation, ties have frayed amid U.S. denunciation of Moscow's aggression in Ukraine and support for Syrian President Bashar Assad.

Anthony Cordesman, a former State Department consultant and now an analyst at the Center for Strategic and International Studies, says Obama has simply seized timely opportunities presented, for example, by the weakening of the Castro government in Cuba and growing Vietnamese concerns about China.

"The truth of the matter is we haven't put anything behind us," Cordesman said. "The legacy a president can leave another president is never binding, and particularly in international affairs it's always a more than one-person game."

___

Reach Josh Lederman on Twitter at http://twitter.com/joshledermanAP. His work can be found at http://bigstory.ap.org/author/josh-lederman