The uncertainty surrounding Syria will hang over the president's three-day overseas trip, which includes a global summit in Russia and a stop in Sweden.
So will Obama's tense relationship with Russian President Vladimir Putin, the world leader who is hosting the Group of 20 gathering and has perhaps done the most to stymie international efforts to oust Syria's President Bashar-al-Assad.
"It's been like watching a slow-moving train wreck for nearly two years," Andrew Kuchins, a Russia expert at the Center for Strategic and International Studies, said of the Obama-Putin relationship. "Mr. Putin and Mr. Obama don't like each other at all. I think there's a deep degree of disrespect."
That's not Obama's only headache as he embarks on the long-planned trip.
The timing of it pulls him away from Washington just as he's urgently seeking to rally lawmakers to support military action in Syria in response to what the administration says was a chemical weapons attack.
And his unexpected announcement over the weekend that he would seek congressional authorization for a military strike against Syria may have stoked doubts among world leaders about his willingness to make good on his threats to rogue nations.
While Syria isn't officially on the agenda at the economy-focused G-20 summit, Obama administration officials say the president sees the gathering as an opportunity to press his counterparts to support military action against the Assad regime.
World leaders also will seek guidance from the US president about whether he plans to proceed with a strike if Congress rejects his proposed resolution - a question Obama's aides have refused to answer.
Votes in the House of Representatives and the Senate are expected next week, just after Obama wraps up his trip.
Republican Sen. John McCain, who has been pushing for stronger action in Syria, said he expected Obama to continue his outreach with Congress even while traveling.
"It's harder when you're overseas," McCain said after meeting with Obama at the White House on Monday, "but he's been manning the phones here the whole time and he'll continue to do that. He's all in on this, obviously."
"We have to make it clear that a vote against this would be catastrophic in its consequences," now and in future international crises, McCain told reporters outside the White House following the hour-long private meeting he and Republican Sen. Lindsey Graham had with Obama.
Obama is to arrive in Stockholm on Wednesday morning after an overnight flight from Washington.
The White House hastily added the Sweden visit to Obama's schedule after he scrapped plans to meet one-on-one with Putin in Moscow ahead of the G-20.
That came in response to the Kremlin granting temporary asylum to National Security Agency leaker Edward Snowden, defying Obama's requests to send the former NSA systems analyst back to the U.S. to face espionage charges.
Snowden's leaks to American and foreign news organizations about secret government spying programs have sparked outrage overseas, particularly in Europe. Obama is likely to face questions about the scope of the programs while overseas, as he did earlier this summer during meetings with the Group of 8 industrial nations.
Even before the Snowden incident, relations between the U.S. and Russia were already on the rocks amid American concerns over human rights and differences on missile defense and nuclear weapons programs.
Putin also has appeared to relish blocking American and Western European efforts to weaken Assad throughout Syria's 2½-year civil war. Russia remains one of Syria's strongest military and economic backers.
In a pointed jab last week, Putin asked Obama to reconsider a military strike, saying he was appealing to Obama not as a world leader, but as a Nobel Peace laureate.
"We have to remember what has happened in the last decades, how many times the United States has been the initiator of armed conflict in different regions of the world," Putin said. "Did this resolve even one problem?"
Administration officials insist the U.S. and Russia can still work productively together during the G-20, though in a slight to Putin, the White House has gone out of its way to characterize the trip as less of a visit to Russia than a trip to the G-20 that happened to be taking place there.
The White House also has ruled out a one-on-one meeting between Obama and Putin on the sidelines of the summit, though the two leaders certainly will spend time together in the larger summit sessions.
Obama is expected to have formal bilateral meetings with other leaders during the two-day summit.
While those meetings are yet to be announced, the president may sit down with counterparts from Britain and France, two nations whose deliberations about Syria have affected his own.
British Prime Minister David Cameron has backed Obama's calls for a retaliatory strike against Syria.
But seeking broader global consensus, Britain pushed for a U.N. Security Council authorization that flopped last week.
A day later, Cameron suffered a stinging humiliation when Britain's Parliament voted against endorsing military action, all but guaranteeing Britain won't play a direct role in any U.S.-led effort.
But France provides Obama an opportunity to show it's not just the U.S. that's convinced it's time to act on Syria.
French President Francois Hollande has said his country can go ahead with a strike, and the French constitution doesn't require such a vote unless and until a military intervention lasts longer than four months.
France's parliament is scheduled to debate the issue Wednesday, but no vote is scheduled.
Obama's stop in Sweden on Wednesday will focus on issues such as climate change, security cooperation and trade.
The trip marks the first time a sitting U.S. president has made a bilateral visit to Sweden.
While in Stockholm, Obama will hold private meetings with Prime Minister Fredrik Reinfeldt and King Carl Gustav, and will break bread with Nordic leaders from Finland, Denmark, Iceland and Norway.
He also will highlight Sweden's technical research programs and celebrate Swedish diplomat Raoul Wallenberg, who is credited for saving at least 20,000 Jews during the Holocaust before mysteriously disappearing after being detained by Soviet authorities near the end of World War II.