U.S. Defense Secretary James Mattis listens as U.S. President Donald Trump speaks to the news media while gathering for a briefing from his senior military leaders in the Cabinet Room at the White House in Washington, U.S., October 23, 2018. Image Credit: Reuters

The abrupt resignation of U.S. Defense Secretary Jim Mattis will reignite concerns from Seoul to Brussels about the stability of American alliances and the direction of policy under an erratic leader.

The former Marine general was widely seen as the last adult in the room, a moderating force against President Donald Trump's suspicion of traditional American alliances and overseas troop commitments. Besides the immediate moves to withdraw forces from Afghanistan and Syria, Mattis's departure could have ramifications for policies toward North Korea, which wants to weaken the U.S.-South Korean alliance, and Europe, where Trump's criticism of NATO has prompted calls to form a continental army.

Here's a look at how the change impacts some global tension points:

Korean Peninsula

Mattis leaves at a perilous time for the U.S.-South Korean alliance, as it grapples with the price of Trump's detente with North Korean leader Kim Jong Un, including his unilateral decision to suspend their annual military exercises. While the president has repeatedly questioned the necessity of having some 28,000 troops on the peninsula and pressed Seoul to pay more for its security, Mattis was a vocal proponent of the alliance.

Mattis's exit - especially after clashing with Trump over troop withdrawals elsewhere - could encourage Kim's efforts to negotiate directly with the president. Just Thursday, North Korean state media published a commentary saying that the removal of the U.S.'s nuclear weapons from the region was a condition of its own disarmament.

Seoul and Washington failed to reach a new deal for sharing the cost of maintaining the U.S. military presence after Trump demanded a sharp increase in contributions from Seoul. "If Trump does not get his way, and as his legal and political peril worsen in the coming weeks, it's not inconceivable that Trump might order a withdrawal of U.S. Forces from Korea," said Daniel Pinkston, a lecturer in international relations at Troy University in Seoul.

Afghanistan

Coming just days after the most recent talks between U.S. representatives and the Afghan Taliban, the timing of Mattis's resignation and the announcement of a partial U.S. troop withdrawal will drastically undercut any efforts underway to end the 17-year-long conflict.

The troop withdrawal will be demoralizing for Afghanistan and a blow to the morale of Afghan security forces, Michael Kugelman, a senior associate for South Asia at the Woodrow Wilson Center in Washington, said via email.

'For the Taliban, it's a best-case scenario. It's a propaganda coup and tactical triumph rolled into one - the Taliban has managed to get U.S. troops to withdraw without giving anything up, and it now has a huge battlefield advantage to look forward to,' Kugelman said. 'Mattis's departure smooths the way for Trump to implement his decision.'

Middle East

Mattis's departure will further unnerve a region already jolted by Trump's decision to withdraw U.S. forces from Syria and his proposal to sell the Patriot missile-defense system to Turkey.

While Trump declared "historic victories" over Daesh, the move to exit Syria drew bipartisan criticism from U.S. lawmakers who warned it leaves the country's future in the hands of Russia and Iran, allies of President Bashar Al Assad.

Turkish President Recep Tayyip Erdogan had earlier threatened to start a military operation targeting America's Kurdish allies, a group known as the YPG, in northeastern Syria. The YPG has been seen as the most effective counter to Islamic State - they will be left to defend the northeast alone after the U.S. withdraws its troops.

Europe

The reshuffle at the Pentagon will only exacerbate already deep concern in the North Atlantic Treaty Organization, which evolved from its Cold War role opposing the Warsaw Pact to support U.S. military action in Afghanistan. After Trump initially failed to recommit the U.S. to the alliance's mutual-defense pact and withdrew from the Paris climate accord, it fell to Mattis to reassure allies: "Bear with us. We will still be there, and we will be there for you."

With him gone, Europe allies will have little confidence there is any resistance within the administration to Trump's "America First" policies. Another influential Cabinet member, Secretary of State Michael Pompeo, has echoed the president's more critical view of the European Union, the United Nations and a host of other international organizations, saying in a Brussels speech this month "they must be reformed or eliminated."

South China Sea

Mattis struck a middle road on one of Asia's biggest potential flash points, the South China Sea. He reassured China's neighbors with more frequent naval patrols and tough rhetoric criticizing Beijing's efforts to expand its military footprint on reclaimed reefs in the disputed water body.

While he dubbed China a "strategic competitor," he advocated engagement between the world's two largest economies, saying "competition doesn't mean hostility." His talks with Chinese Defense Minister General Wei Fenghe helped ease tensions after a near collision in the South China Sea, leading to a Washington meeting in which both sides agreed the "military-to-military relationship could be a stabilizing factor" for ties.

"The Chinese side in general thinks Mattis is a rational and cautious person and is worrying about who will replace him," said Sun Zhe, co-director of the China Initiative at Columbia University. "Under Mattis, U.S. and China armies have some contradictions in the South China Sea and arms sales to Taiwan, but the conflict has not escalated and mutual exchanges were continuing."

Japan

Over the long history of postwar disputes between the U.S. and Japan, their security pact has often proved an anchor in the relationship. Trump threw that into doubt by not only threatening to levy auto tariffs against Japan, but questioning its contribution to security. Since taking the job, Mattis has repeatedly visited to share reassuring words about the alliance with Prime Minister Shinzo Abe.

"When the Trump administration was straying off course, it's not an exaggeration to say that it was Mattis who just about managed to keep foreign policy to the proper line," Akihisa Nagashima, a former vice defense minister of Japan and now an opposition lawmaker without party affiliation, said on Twitter. "The outlook beyond February next year is unclear."