Secretary of State John Kerry is on a one-man global mission impossible.

In nearly three decades in Washington and during his unsuccessful 2004 run for president, the senator from Massachusetts often appeared overly cautious, detached and robotic, looking and playing the part of the elder statesman but lacking the passion to take tough stands that could alienate one side or another.

Now the nation's top diplomat in the twilight of his career, Kerry, 70, has dropped any reluctance to go all in, hoping to leave his mark and redefine America’s engagement with the world.

Instead of pivoting away from the Middle East as President Obama repeatedly has tried to do, Kerry is seeking a peace agreement between Israel and the Palestinian Authority -- the elusive holy grail of U.S. diplomacy.

Kerry has also assumed the burden of the Mideast’s political turmoil and sectarian violence.

Syria's brutal three-year civil war continues to horrify and frustrate the West as the death toll has risen to 140,000. Close allies and congressional lawmakers fear that easing sanctions on Iran for six months will give the rogue nation an economic boost while failing to impede its nuclear program. Meanwhile, al Qaeda is reasserting itself in Iraq as Baghdad teeters toward civil war, and Afghanistan's mercurial President Hamid Karzai continues to rebuff calls to sign a post-war security agreement.

Adding to that full plate, Kerry is being forced to manage emerging flash points around the globe -- from the violence in Ukraine to rising tensions between China and Japan.

Foreign policy watchers say that the secretary of state’s efforts are complicated by President Obama, who critics accuse of being disengaged and focused more on the income inequality and saving his health care law.

Some say Kerry is alone within the administration in seeing the big geopolitical picture, even as he struggles to keep up.

Kerry, they note, is taking a tougher stand against Russia, criticizing its role in Syria and Ukraine, even as Obama takes a hands-off approach and insists that Moscow's opposition to U.S. policy is not part of “some Cold War chessboard.”

Kerry, though, is not backing away from his grand ambitions in the Middle East, experts and former diplomats say, even as he is forced to juggle the myriad demands of his job.

In classic Kerry style, he speaks in long-winded metaphors about the challenge. While traveling to Jerusalem in early January, he described the Israeli-Palestinian talks as an elaborate 3,000-piece jigsaw puzzle.

“In the end, all of these core issues fit together like a mosaic, like a puzzle and you can't separate out one piece or another,” he said. “The last pieces may decide to fall into place, or may fall on the floor, and leave the puzzle unfinished.”

Kerry has shuttled to the region 14 times — more than half of the 26 overseas trips he's taken in a little more than a year on the job — according to a running tally on the State Department's website.

“He has tremendous confidence in his capabilities to bring changes about – even when that confidence collides with the realities,” said Faysal Itani, a fellow with the Atlantic Council, a centrist foreign policy think tank.

Even after the Benghazi debacle, Washington's liberal foreign policy community marveled at the dedication of Kerry's predecessor, Hillary Clinton -- her hours logged overseas and reputed diplomatic skill. Already, though, Kerry's go-big approach is making Clinton's tenure look restrained and risk-averse.

Conservatives collectively rolled their eyes when Kerry, speaking to college students in Jakarta recently, claimed that climate change was among the world's most serious problems, including terrorism, poverty and weapons of mass destruction. The statement prompted former House Speaker Newt Gingrich to call for his resignation.

But even the president's biggest foreign policy detractors give Kerry an “A” for effort.

“John is trying as hard as he can to fly the American flag in a productive way but the policies of this administration in Syria, the president's insistence of no new sanctions in Iran, make it very difficult,” said Sen. Lindsey Graham, R-S.C.

“Kerry is not commander in chief — so he doesn't even have a pair of twos if this is a poker game,” he said.

On Syria, Graham and Sen. John McCain, R-Ariz., the tag-team critics of Obama's foreign policy, say the president's zig-zags -- he failed to arm rebel groups early on and backtracked from the threat of military action -- is allowing President Bashar Assad to win the bloody civil war.

But both are careful not to fault Kerry, their former longtime Senate colleague, for what they view as the mistakes of his boss.

In fact, the GOP senators see Kerry as an ally, claiming that he too is frustrated by Obama's policies and has been pushing the president to start arming the Syrian opposition with surface-to-air missiles and paying its fighters.

McCain and Graham recently met with Kerry behind closed doors and told reporters afterward how deeply disappointed he is with the administration's continued reluctance to up its game in Syria, where Assad is dropping barrel bombs with abandon, killing thousands of civilians. Syria has become a magnet for al Qaeda fighters, to the extent that intelligence sources call their presence there a direct threat to the U.S. homeland.

Hezbollah and the Russians are all in and we're just twiddling our thumbs about what to do about al Qaeda,” Graham said. “I talked to Secretary Kerry and I think he understands that the al Qaeda situation has changed dramatically but you can have a Geneva [conference] two, four, six, eight, nine, 10 [times] and you're not ever going to go anywhere as long as Assad thinks he's winning.”

The State Department denied that Kerry's frustration with Syria is directed at the president's policy, but the White House also concedes the Geneva peace talks failed to produce a solution.

Republicans are far more critical of Kerry's handling of the negotiations with Iran and the interim nuclear deal that the U.S. and five other world powers struck last fall.

Sen. Mark Kirk, R-Ill., who co-authored a new Iran sanctions bill with Foreign Relations Committee Chairman Bob Menendez, D-N.J., that Obama vows to veto, was blunt: “I would say that he largely got taken to the cleaners by the Iranians in this agreement.”

Others say the shuttle diplomacy and round-after-round of peace talks cost Kerry foreign policy successes elsewhere, pointing to Ukraine, where the U.S. failed to enact tough sanctions on President Viktor Yanukovych early in his crackdown on opposition protesters, hoping instead that the European Union would take the lead.

Some seasoned foreign policy hands, though, are more charitable in assessing Kerry's pursuit of Syrian peace and a long-term deal with Iran.

Anthony Cordesman, who holds the Arleigh A. Burke Chair in Strategy at the Center for Strategic and International Security Studies, suggested that by pursuing the Geneva talks Kerry was building a foundation for a tougher approach on Syria.

“We wouldn't have gained anything if we didn't have talks at Geneva,” said Cordesman, noting that he's “no fan” of how Obama handled the Syria situation. “For all of its limits, it at least showed the world where the two sides stand and provided at least some basis for a common approach for U.S. and Europe and allied Arab states.”

Faced with few options on Syria, the administration's decision to let the Geneva talks play out showed the world that the United States was exhausting its diplomatic options before reintroducing the threat of military action, he said.

On Iran, Cordesman said, it is difficult for naysayers to attack Kerry’s efforts.

“If we had done nothing, Iran would have moved to nuclear capability and we won't know until June 20th whether we have a sustainable deal with Iran,” he said, referring to the end of the six-month interim agreement.

Critics question if Kerry’s broad approach will succeed. Jeffrey White, a defense fellow at the Washington Institute for Near East Policy, said Kerry's furious Middle East efforts should not be confused with achieving a minor success.

“We tend to confuse activity with progress… If you fly around and have a lot of meetings that that's actually accomplishing something. We often find out that it isn't,” he said.

As Kerry tries to manage international crises and press Mideast peace talks, he still holds a strong reservoir of goodwill among his former Senate colleagues.

Sen. Tim Kaine, a Virginia Democrat who leads the Foreign Relations subcommittee on the Near East, South and Central Asia, rejected criticism that Kerry's pursuit of an Israeli-Palestinian accord is ill-timed and fruitless.

Kaine said that envoys from both sides are “uniform in their praise of Secretary Kerry … They said they are so glad he's deeply engaged.”

Graham, though, acknowledged that Kerry has a tough task in front of him.

“John is trying to destroy the narrative singlehandedly that America is disengaged in the world, but he's just one man,” he said.