Here's a look at how the abrupt resignation of US Defence Secretary James Mattis and the decision to withdraw US troops from the Middle East impact other global powers and key tension points:

KOREAN PENINSULA

Mr Mattis leaves at a perilous time for South Korea, which is grappling with the price of US President Donald Trump's outreach to North Korean leader Kim Jong Un and his decision to suspend joint annual military exercises with the South.

Mr Mattis had worked to reassure South Korea that US commitment remained strong, while the President focused on questioning the necessity of having some 28,000 troops on the peninsula and pressed Seoul to pay more for its security.

His exit could encourage Mr Kim's efforts to negotiate a reduced American presence directly with the President.

AFGHANISTAN

Mr Mattis' resignation and the announcement of a partial US troop withdrawal undercut recent efforts to finally end the 17-year-long conflict in Afghanistan. They have also left US allies hanging. Germany, which has the second-biggest troop contingent in Nato's Afghanistan mission after the US, is particularly miffed.

Mr Trump's troop withdrawal will be demoralising for Afghan security forces and boost morale among their enemies, Mr Michael Kugelman, a senior associate for South Asia at the Woodrow Wilson Centre in Washington, said via e-mail.

MIDDLE EAST

Even before the decision to withdraw US forces from Syria, the Middle East had been jolted by a series of policy about-faces under Mr Trump, from the undoing of his predecessor's signature deal with Iran to the decision to move the US embassy in Israel to Jerusalem.

US lawmakers warn that the US withdrawal from Syria leaves the country's future in the hands of Russia and Iran, the primary backers of President Bashar al-Assad.

EUROPE

The reshuffle at the Pentagon will exacerbate already deep concern in Europe and, in particular, Nato. After Mr Trump initially failed to recommit the US to the alliance's mutual-defence pact and withdrew from the Paris climate accord, it fell to Mr Mattis to reassure allies.

With him gone, allies are losing confidence that there's any resistance within the administration to Mr Trump's "America First" policies.

Mr Guy Verhofstadt, head of the Alliance of Liberals and Democrats for Europe in the European Parliament and a former Belgian prime minister, said: "Europe stands unprepared and needs to speed up the establishment of a European Defence Community."

RUSSIA

For the Kremlin, Mr Mattis' announcement capped a sweep of wins the Kremlin had long desired but did not believe would ever happen. But some were worried about what the US retreat might mean for conflict zones where Russia is now left as the primary foreign power.

Kremlin spokesman Dmitry Peskov warned that the unpredictability of US policy was a growing risk.

SOUTH CHINA SEA

Mr Mattis struck a middle road on one of Asia's biggest potential flashpoints, the South China Sea. He reassured China's neighbours with more frequent naval patrols and tough rhetoric criticising Beijing's efforts to expand its military footprint on reclaimed reefs. Yet his talks with Chinese Defence Minister General Wei Fenghe helped ease tensions after a near collision in the disputed waters.

"The Chinese side in general thinks Mattis is a rational and cautious person and is worrying about who will replace him," said Dr Sun Zhe, co-director of the China Initiative at Columbia University.

BLOOMBERG