WASHINGTON (Reuters) - From Syria's bloody civil war and the slow-burning crisis in eastern Ukraine to perennial diplomatic headaches such as the North Korean and Iranian nuclear programs, Rex Tillerson would face a messy, complex world as the chief U.S. diplomat.
Below are brief descriptions of some of the problems that will land on his plate if he is confirmed by the U.S. Senate as the 69th U.S. secretary of state after a career spent at Exxon Mobil Corp (XOM.N), where he rose to be chief executive.
Tillerson will inherit an increasingly complex conflict in Syria, where the rebel-held eastern portion of Aleppo is on the verge of falling to Syrian government forces backed by Russia, Iran and Shi'ite militias from Lebanon and Iraq.
The civil war, now well into its sixth year, pits President Bashar al-Assad against a range of rebel groups, many supported by outside powers including the United States, Turkey and Gulf states such as Saudi Arabia and Qatar.
During the U.S. presidential campaign, President-elect Donald Trump suggested his top priority was defeating the Islamic State militant group that has launched attacks in Europe and inspired mass-casualty attacks in the United States.
If Trump cooperates with Russia against Islamic State, which holds a swath of northeastern Syria, there is a risk that moderate rebels, angered by what they viewed as abandonment by the United States, could gravitate toward militant Islamist factions that pose a potential threat to Western interests.
Moreover, the persistent bloodshed could unleash new and destabilising waves of refugees into neighbouring nations from which they could then attempt to reach Europe. Earlier refugee flows triggered a backlash that has bolstered far-right nationalist parties in Europe.
Cooperating with Russia would cause other complications for the new U.S. administration. For one thing, it could, by extension, align the United States with Iran and its Lebanese proxy militia, Hezbollah. Iran has used its ties to Syria to project force in the region, a troubling fact for Israeli officials not eager to see Tehran extend its influence.
Next door in Iraq, Tillerson will confront a country struggling to overcome deep ethnic and sectarian divisions as it fights to stamp out the Islamic State insurgency with the help of some 6,000 U.S. support troops and military advisers.
Even if Iraqi security forces prevail in retaking the Islamic State stronghold of Mosul, the country’s second-largest city, the extremist group is expected to revert to waging a guerrilla war against the Shi'ite-dominated central government.
If so, that could force Trump and Tillerson to decide whether the United States should remain engaged - and attempt to counterbalance the powerful influence of neighbouring Shi'ite-led Iran – or play a diminished role.
Tillerson also may face a renewed drive by Iraq’s minority Kurds, trained and armed by the United States to fight Islamic State, for independence for their oil-rich autonomous northern region.
Such a move, which the United States has opposed, would raise the threat of a conflict between the Kurds and Baghdad that could affect the international petroleum market. It could also fuel demands by Kurds in Syria and in Turkey for greater autonomy, a prospect that Turkish officials bitterly oppose.
North Korea's secretive, authoritarian government has conducted five nuclear tests over the past decade, four of them during the Obama administration and two this year alone, along with an unprecedented flurry of ballistic missile tests.
The North's tests have brought tighter U.N. Security Council sanctions, including some passed last month that aim to cut its export earnings by a quarter, but no sign that Pyongyang's young leader, Kim Jong-un, is willing to restrain its nuclear and missile programs.
Reclusive, impoverished North Korea poses a direct threat to rich, democratic South Korea, a U.S. treaty ally that technically remains in a state of war with Pyongyang because the 1950-53 Korean War ended in a truce rather than a peace treaty.
The North regularly threatens to destroy the South and the United States, which maintains about 28,000 troops in South Korea as a tripwire to deter North Korean from attacking.
North Korea said after its September nuclear test that it had mastered the ability to mount a warhead on a ballistic missile. If it developed an effective intercontinental ballistic missile, or ICBM, it could strike the continental United States.
Trump has harshly criticized the agreement struck on July 14, 2015, between Iran and six major powers under which Tehran agreed to restrain its nuclear programme in exchange for relief from economic sanctions.
Many Republicans, particularly in the U.S. Congress, have argued that the deal, negotiated by the United States, Britain, China, France, Germany and Russia, should be torn up.
While Trump has been critical of the agreement, it is not certain he would actually abrogate it.
For one thing, Israeli officials quietly say that they prefer to have Iran live under the deal's restraints and subject to its transparency measures, than totally freed from both, which could allow them to race to acquire nuclear weapons.
Further, if the United States were to be blamed for the agreement's collapse, it would be unlikely to find much sympathy among its European allies to reimpose sanctions on Tehran.
Even if the deal holds, Tillerson may have his hands full trying to find ways to counter Iranian support for Syria's Assad, Hezbollah in Lebanon and the Houthis, the militia fighting the Saudi-backed government in Yemen.
Tillerson may quickly find himself dealing with a crisis of Trump's own making with China given Beijing's angry response to the Republican president-elect's view that Washington did not necessarily have to stick to its long-standing position that Taiwan is part of "one China."
If he were to upend some four decades of settled U.S. policy, Trump risks antagonizing Beijing, which could decide to restrict cooperation with the United States or, if truly pushed, seek to subvert U.S. policy aims around the world.
The United States has a vast array of interests with China, which is the third-largest destination for exports of U.S. goods and the largest source of goods imported into the U.S. market. China is also the largest holder of U.S. government debt.
Among the areas where the two sides have worked together under the Obama administration are maintaining growth in the world economy, reining in the Iranian nuclear programme and seeking to combat climate change. They have also had disagreements over North Korea and China's maritime claims.
Russia seized the majority Russian-speaking Crimea from Ukraine on March 16, 2014, after an uprising toppled Ukraine's pro-Russian president.
Ukraine, the United States and many governments in Western Europe also say that Moscow has armed and encouraged pro-Russian separatists in eastern Ukraine, and covertly infiltrated Russian military personnel.
The conflict between Ukrainian forces and Russian-backed separatists has cost nearly 10,000 lives since 2014. Germany and France have tried with little success to convince both sides to implement a peace deal agreed to in Minsk last year.
The Kremlin denies Western charges that it is stoking the separatist movement and aiding the rebels and accuses Ukraine of perpetuating the violence and violating the Minsk deal.
The United States and the European Union have imposed a range of economic sanctions on Russia as a result of its annexation of Crimea and its actions in eastern Ukraine.
Trump has said that better relations with Russia could be a great benefit to the United States and there are suspicions that he and his nominee for secretary of state might be inclined to ease the Western sanctions on Russia.
One U.S. official said there might be a deal to be done if Russian President Vladimir Putin were willing to abide by the Minsk peace deal, or to agree to something similar under a different name, in return for sanctions relief.
Trump has given few specifics on his plans for Afghanistan, where nearly 10,000 U.S. troops remain more than 15 years after the Islamist Taliban were toppled by U.S.-backed Afghan forces.
Trump has said, however, that the United States should stop carrying out "nation building," something it has been seeking to do for a decade and a half in the southwestern Asian nation.
One of the most important questions facing Trump on Afghanistan, former officials and experts say, is how many U.S. troops will stay on there.
Acknowledging that Afghan security remained precarious and Taliban forces had gained ground in some places, President Barack Obama shelved plans to cut the U.S. presence almost in half by year's end, opting instead to keep 8,400 troops there through to the end of his presidency in January.
A number of provincial capitals have been under pressure from the Taliban, while Afghan forces have been suffering high casualty rates, with more than 5,500 killed in the first eight months of 2016.
Reporting by Jonathan Landay and Phil Stewart in Washington; Additional reporting by Madeline Chambers, Michael Nienaber and Jean-Baptiste Vey in Paris; Writing by Arshad Mohammed and Jonathan Landay; Editing by Peter Cooney