Across the country, families will gather to celebrate, Christmas this year falling on the same weekend as the first day of Hanukkah. Charities will hold events to make sure the needy and homeless have a meal to share. Many stores will go quiet.

For a brief moment, it may seem, as the Christmas song wistfully observes, “Our troubles will be miles away.”

Yet the brief pause of this holiday season comes at a time when the globe is again being shaken by changes that suggest a perilous 2017. The expectations, agreements and established orders that provide international stability are under stress.

Entering the world stage as the leader of the world’s most powerful and well-armed nation is a new, untested president with no governmental experience. In the campaign, Donald Trump’s impulsiveness and unpredictability proved attractive to many voters. As president, these traits could lead to international disputes, or turn a problem into a crisis.

Trump has alternately suggested an isolationist, “America first” foreign policy posture, and a highly aggressive military approach to crush any enemy that would threaten the nation.

Perhaps a President Trump will be the deal maker that candidate Trump promised. Maybe his suggested willingness to use the big stick will provide the space for negotiations, his unpredictability keeping foes off guard. The fact is no one knows what to expect, adding to global uncertainty.

The threats are clear.

Next year marks the centenary of the Bolshevik revolution. The occasion finds Russia led by an increasingly aggressive Vladimir Putin. His economy weakened by depressed oil prices and rampant corruption, Putin has pursued a strategy of fueling nationalism with foreign exploits. He has invaded Ukraine and threatened other old Soviet satellite nations. He vows to strengthen his nation's nuclear capability and used brutal Russian airpower to extinguish the rebellion against his Syrian ally and fellow strongman, President al-Assad.

A dispirited Europe, its economy never having fully recovered from the Great Recession, its cities targeted by attacks from radical Islamic terrorists and roiled by Mideastern refugees, has seen nationalist movements emerge, challenging the European Union and the solidity it provides. Without U.S. leadership, this Europe could well shrink from a belligerent Russian bear.

In 2016, Great Britain voted to withdraw from the EU, a move that seemed to even shock many of those who voted in favor. In 2017, the world will learn what Brexit looks like and what follows next.

In France, the ascendant far-right National Front and its charismatic presidential candidate, Marine Le Pen, could shake up the presidential elections this spring and the vote for Parliament in June. A National Front victory, and its likely call for France to also vote on an EU exit, remains a long shot, but then so was a certain U.S. presidential candidate.

German Chancellor Angela Merkel, first elected in 2005, has provided Europe’s most consistent leadership. Facing criticism of her liberal policies in accepting Syrian refugees, and over the terror attacks that followed, she could face her first serious political challenge if she chooses to seek re-election in 2017.

In the Middle East, the Islamic State has seen its territorial gains rolled back. Its response, however, could be more terror attacks on the West. The tribal divisions in Iraq — Shiites, Sunnis and Kurds — have quieted with the Islamic State as their common enemy, but will re-emerge. In fact, the Sunni-Shiite divide remains a low-boil civil war across the region, often fought by surrogates and always ready to boil over.

China, meanwhile, is sure to test a Trump administration both geopolitically and economically. Expect it to continue to make its case for dominance in the South China Sea, using the pretext of its manmade islands and ignoring international rulings to the contrary. With the U.S. abandoning the Trans-Pacific Partnership trade deal, Beijing will seek its own deals to extend its economic dominance in Asia as well.

A lot will be on Mr. Trump’s plate, and not just for Christmas dinner.