海角大神

The upheaval and terrible violence in Syria are just one sign. The same thing is happening elsewhere: not only in the Middle East, but as far afield as Eastern Europe and East Asia.

We are seeing the beginnings of an answer to a question with potentially critical implications: What happens when a superpower decides it no longer wants to be a superpower?

The United States remains the world鈥檚 preeminent economic and military power. Only one other country, China, harbors realistic ambitions to displace it from that position. And the U.S. is not suddenly going to shrink into the geopolitical equivalent of Nepal, or Luxembourg, or Paraguay.

Yet its status as a superpower has been rooted in something more than mere economic or military muscle. It has rested on political and diplomatic leadership, and on a web of alliances around the world, formal and informal, that has magnified U.S. influence. The result: Even in areas of conflict where Washington has not directly sought to intervene, its mere presence has exercised a major influence. Its voice has mattered, and it has been listened to.聽

That鈥檚 what is changing under President Donald Trump, with Syria providing the most dramatic evidence.

In Syria, it was Mr. Trump鈥檚 predecessor, Barack Obama, who decided against significant U.S. military involvement in the country鈥檚 civil war. But in his determination to roll back the so-called Islamic State, President Obama did draw on America鈥檚 superpower toolbox. The U.S. was critical in assembling an international partnership of more than 70 countries to take on the Islamic State group. And militarily, it forged a battlefield alliance with Kurdish fighters, backing them with money, weapons, training, and a small contingent of special-forces troops.

With ISIS beaten back, at least territorially, the situation in Syria until this month had settled into a delicate balance of power. The regime of President Bashar al-Assad, backed by Russian forces, was gradually moving to control more of the country. President Recep Tayyip Erdo臒an of Turkey, to the north, continued his monthslong public insistence that northern Syria be cleared of the US-allied Kurds.

In pure military terms, there was little to stop Turkey: fewer than 2,000 U.S. soldiers on the ground. But for Mr. Erdo臒an, and for Mr. Assad and Russian forces as well, it was not the troop numbers that forestalled any advance. It was the insignia on their uniforms: the potential price of taking on the whole range of measures available to the U.S. as a superpower. It was the U.S. presence that mattered.

Once the Trump administration signaled it no longer cared to remain, Mr. Erdo臒an鈥檚 army, reportedly backed by jihadist irregulars, surged into Syria. Mr. Assad鈥檚 troops pushed northward as well into the roughly one-third of the country controlled by the U.S.-backed Kurds. And while what comes next is uncertain 鈥� Iran also has forces in Syria 鈥� one thing is clear: All diplomatic roads now lead through Russia, where Mr. Erdo臒an has now traveled for talks with President Vladimir Putin.聽

Washington鈥檚 fairly low-key response to last month鈥檚 Iranian attack on key Saudi oil facilities was seen in the region as conveying a similar message, prompting the Saudis to explore their own options for de-escalation with Tehran, and also to welcome Mr. Putin for talks.

But it鈥檚 not just in the Middle East that the absence 鈥� or at least the quieting 鈥� of America鈥檚 voice seems to be having an impact. In Asia, Japan and South Korea, two of Washington鈥檚 key allies, have been locked in a political battle, escalating into a trade war. At issue is a deeply felt enmity dating from Japan鈥檚 often-brutal colonial rule over the Korean Peninsula in the first half of the last century.

That鈥檚 not something any outside power can simply wish away. But the U.S. has, over decades, played a critical role in damping down such crises between allies. That鈥檚 now made more complicated by a similar retreat from engagement in Asia: first, at the start of the Trump administration, by withdrawing from the Trans-Pacific Partnership, an alliance set up with U.S. backing to balance the expanding influence of China; and more recently, by calling into question the value of continued U.S. troop deployments in South Korea.

In Europe, there鈥檚 similar concern over Mr. Trump鈥檚 questioning of the importance of the NATO military alliance, and of the implications that might have, for instance, in Ukraine, where Mr. Putin has annexed Crimea. At issue, as in Syria, is not whether there should be U.S. forces on the ground to forestall further advances there or elsewhere in Eastern Europe. It is the deterrent importance of America鈥檚 superpower commitment, its political and diplomatic presence, and its voice.

The longer-term question is whether there might be a return to American superpower engagement. Mr. Trump seems to enjoy broad support on at least one issue: That in the wake of the long and still-unresolved war in Iraq, the U.S. should steer clear of major military operations overseas. But the strong bipartisan criticism of his pullout from Syria, especially over the evident gains for Messrs.聽Erdo臒an, Assad, and Putin, suggests that the effects of a more fundamental disengagement could be causing alarm.