Hailing Robert Gates for his "profound sense of duty" and integrity and his four decades of public service, President Obama used a farewell tribute at the Pentagon to surprise the outgoing defense secretary with the Presidential Medal of Freedom.

The medal is the highest honor a president can bestow on a civilian and is presented to people "who have made especially meritorious contributions to the security or national interests of the United States, to world peace, or to cultural or other significant public or private endeavors," according to information provided by the White House.

The citation accompanying the medal called Gates a champion of servicemembers and their families and noted what it called his "unwavering patriotism." He retires after having served eight presidents.

"Bob, today you're not only one of the longest-serving secretaries of defense in American history, but it is also clear that you've been one of the best," Obama said.

Gates is the only defense secretary to have served both a Republican Commander-in-Chief, President George W. Bush, and a Democrat -- something the president made a point of highlighting.

"In his willingness to become the first secretary of defense to serve under presidents of both parties, the integrity of Bob Gates is also a reminder -- especially to folks here in Washington -- that civility and respectful discourse and citizenship over partisanship are not quaint relics of a bygone era," he said.

Gates' departure comes as the U.S. is engaged in conflicts in Afghanistan, Libya and Iraq -- even though the Obama administration has removed 100,000 combat troops from Iraq. June has been the deadliest month for American troops in the country in two years.

In remarks before introducing the president, Joint Chiefs Chairman Adm. Mike Mullen praised Gates for his honesty, pragmatism and grit, for his "staunch commitment to truth telling" no matter how uncomfortable that truth might be or how high or low on the chain of command it needed to travel and for his leadership during the transition to a new mission in Iraq, the transition "we now strive for in Afghanistan" and his efforts to save taxpayer dollars by tackling inefficiencies in the Pentagon budget.

Obama echoed many of those sentiments and spoke of Gates having made it his mission to make sure his department was doing all it could to protect U.S. troops, including providing mine-resistant vehicles and reducing the time it takes to evacuate injured troops from the battlefields of Afghanistan.

Upon receiving the Medal of Freedom, Gates thanked Obama for his confidence in taking "the historic step" of asking him -- "someone he did not know at all" -- to serve as his defense secretary. He also sparked laughs by making a joke that referenced the May raid that killed Osama bin Laden.

"I'm deeply honored and moved by your presentation of this award," Gates told the president. "It is a big surprise, but we should have known a couple of months ago you're getting pretty good at this covert-ops stuff."

Outgoing CIA Director Leon Panetta was confirmed unanimously by the Senate earlier this month to take over as Pentagon chief.