"We want Pakistan to be a partner in this effort," said Sen. John Kerry, D-Mass., of the war against al Qaeda and the Taliban, before adding: "We believe there are things that can be done better."

The chairman of the Senate Foreign Relations Committee was speaking to reporters in Mazar-i-Sharif, Afghanistan, last Saturday en route to an intense 24-hour visit to Islamabad.

There -- where relations with the United States have frayed nearly to the breaking point and the lanky lawmaker from Massachusetts is seen as one of the most reasonable of American politicians -- Kerry once again performed his one-man opera of partnership and disappointment, this time for the benefit of Pakistan’s president, prime minister, and army chief of staff.

Among the immediate signs that his audience appreciates him: the Pakistanis' transfer to U.S. custody, the day after Kerry's visit, of the helicopter tail they found at the compound where Usama bin Laden lived and died. Yet Kerry’s twin messages, alternately encouraging and reproachful, were vintage stuff to longtime watchers of his work: stroking and prodding a troubled ally, a stab at engagement with a dose of realism.

After more than a quarter-century in the Senate, the decorated Vietnam War veteran and antiwar figure, now sixty-seven years old, has emerged as one of his generation’s “wise men” of Beltway foreign policy and national security. Perhaps the most well traveled member of the 112th Congress, Kerry has pursued an unabashedly activist chairmanship. Accordingly, his voice, on all manner of transnational threats and crises, commands respect in the White House, the State Department, and every foreign capital. Were President Obama to win a second term, many envision Kerry as the front-runner to succeed Hillary Clinton as secretary of state.

“He's a powerful chairman in a long line of powerful chairmen,” says Stephen Hess, the senior fellow emeritus at the Brookings Institution. “When he went to Afghanistan and to Pakistan…he was doing something beyond what is the role and function of the chairman of the Foreign Relations Committee….A member of Congress goes [abroad] as a fact-finder. They ask questions. They listen and they come back. They don't go to make policy….[Kerry] did.”

Yet the extraordinary overseas events of the first half of 2011 – the revolutions of the Arab Spring in the Middle East, the detention of an American CIA agent in Pakistan on murder charges, and the covert operation that resulted in the killing of bin Laden in Abbottabad – have exposed, in equal measures, the power and perils of Kerry’s activist approach to his job.

After all, it was Kerry who, early in 2009, led Congress on the charge to approve a five-year, $7.5 billion aid package to Pakistan. Now, with speculation swirling that rogue elements within Pakistan’s intelligence service may have worked actively to help bin Laden elude his American pursuers, Kerry acknowledged to reporters in Kabul on Sunday: “There are calls, in some quarters in Congress, for a shift in the aid program, unless there is an improvement in the current situation."

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What’s more, Kerry returned from Pakistan just in time to hear the Obama administration’s announcement of new sanctions against Syrian President Bashar al-Assad and six of his chief lieutenants. The move to freeze the assets of Assad and his men came as the Syrian regime intensified its crackdown on pro-democracy protests across the country. Human rights groups say the clashes have seen security forces kill more than 850 people.

Since taking over the Foreign Relations Committee in December 2008, Kerry has traveled to Syria four times, and met personally with Assad six times – including an intimate dinner featuring the two men and their wives. That Kerry came away from these encounters believing strongly in the prospect of improved relations with Damascus is undeniable.

As late as March 16, in an appearance before the Carnegie Endowment for International Peace in Washington, Kerry expressed optimism that Assad – a close ally of Iran and major benefactor of Hamas and Hezbollah – would begin to reform his autocratic government. The first “Day of Rage” in the current wave of Syrian protests had been staged February 4.

"My judgment is that Syria will move, Syria will change, as it embraces a legitimate relationship with the United States and the West and the economic opportunity that comes with it,” Kerry told the Carnegie audience. “President Assad has been very generous with me, in terms of the discussions that we have had….I asked President Assad to do certain things to build the relationship with the United States, and sort of show the good faith that would help us to move the process forward....I gave him five or six requests....Guess what? All six were done, delivered. I think it's incumbent on us to move that relationship forward in the same way.”

Within forty-eight hours, according to the BBC and other news outlets, Assad’s security forces opened fire on thousands of protesters with live bullets, killing at least five people and injuring “hundreds” more. Tommy Vietor, a spokesman for the National Security Council, declared sternly that “those responsible for today's violence must be held accountable.” Little talk could be heard, at the White House or the State Department, of Syria’s inevitable movement toward change, or of Assad’s “generosity” with visiting lawmakers.

Senator Kerry was fooled by Bashar al-Assad,” says Elliott Abrams, a senior fellow at the Council on Foreign Relations and senior official in the Reagan and George W. Bush administrations. “He bought the line. He just bought it hook, line and sinker. He went over he had several meetings [where] Assad said, ‘I want to reform and I want to change,’ and then they all went out to dinner....Meanwhile, the political prisoners in Syrian's prisons were being tortured, and nothing changed, and there was zero reform. So I just think that one has to say that Senator Kerry became, in a sense, a tool of Assad in misleading the American government, the American press and the American people.”

Sen. Kerry declined to be interviewed for this article. So did Sen. Richard Lugar, the Indiana Republican who formerly served as chairman and is now the committee’s ranking Republican.

Hess, who advised presidents from Eisenhower to Carter, told Fox News he considered Kerry a “perfectly logical” candidate to be considered for the job of secretary of state, the nation’s top diplomat. He likened Kerry’s influence as chairman of the Senate Foreign Relations Committee to that of the late Sen. J. William Fulbright, the Arkansas Democrat whose controversial hearings helped sway popular opinion against the Vietnam War in which Kerry served. “[Kerry] is in an important place at an important time,” Hess said, adding that that “has not always been the case with the chairman of Foreign Relations.”

Still, Hess agreed Kerry had been “wrong” to invest such hopes in Assad, saying Kerry had “chosen to be out front in trying to improve relations with a dictator in Syria. Those are now such damaged goods that I assume he would have to do a mea culpa at some point, and otherwise explain what he saw, and what has changed….Clearly, being too close to a dictator is not the place an American official wants to be.”

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In the years since he lost the 2004 presidential election, Kerry has undeniably regained considerable stature as a senior man the Senate, a globe-trotting diplomat with every portfolio. Some have spoken of his work during the Obama presidency as a kind of “audition” for the secretary of state’s job.

Those close to Kerry dispute such notions, saying he is only trying to play constructive roles where he can, always in support of – never at odds with – U.S. policies under the Obama administration. And Kerry himself has disputed the idea that he misjudged Bashar al-Assad’s propensity for reform. “I've always said the top goal of Assad is to perpetuate his own regime,” Kerry told Foreign Policy magazine on May 10. “I didn't hold out hope. I said there were a series of things that, if he engaged in them, there was a chance he would be able to produce a different paradigm. But he didn't."

Both Kerry’s detractors and defenders point to his style as a decisive factor in the substantive policies he helps implement – and craft – as chairman.

“The approach to foreign policy that he seems to adhere to mostly is a personal foreign policy,” says Abrams, a deputy national security adviser from 2005 to 2009. “That is one that counts a great deal on the role of the individual…to deal face to face with foreign leaders. And he's done that. He's traveled a great deal and he's met, I think it's fair to say, every important foreign leader.”

Simon Rosenberg, a former Clinton-Gore operative who is now president of NDN, a Washington think tank, says Kerry enjoys a “very close and collaborative relationship” with Secretary Clinton.

“He’s defining his role as a very vigorous and engaged one,” Rosenberg told Fox News. “I think it's the right tone for the moment we're in…The world is changing so much; things are happening very quickly. Whether it's the rise of China or the Arab Spring or the way that Latin America's growing, we're entering a period of dramatic global change and I think it requires more active leadership…So I think he's the right guy for the right time.”

Craig Caruana and Anne Marie Riha contributed to this report.