The link announced by Belgian officials Sunday between the Islamic State militants responsible for November's terrorist attack on Paris and those involved in last month's attacks in Brussels represents a most unwelcome development, signaling that the group's network in Europe may be far more extensive than once thought. Counterterrorism officials now must work on the assumption that similar cells exist in other European countries as well, including Germany and the U.K. Moreover, the threat will only get worse the longer ISIS occupies large swaths of Syria and Iraq where it can train foreign-born fighters and send them back to their home countries to plot future attacks.

Though investigators recently have made progress in identifying and apprehending ISIS militants in Belgium, their success has been limited because this isn't a problem that can be solved in Europe. The fact that ISIS can export hundreds of militants from its bases in Iraq and Syria to strike targets in Europe means that any serious response eventually must involve retaking the territories ISIS occupies and shutting down the pipeline through which militants infiltrate Europe. Yet no country is willing to attempt that amid the chaos created by Syria's five-year civil war.

The war has killed more than a quarter-million people, displaced millions more from their homes and driven an enormous refugee crisis that threatens to destabilize Europe's post-Cold War political order. But as long as the key to stopping it remains in the hands of Syrian President Bashar Assad and his enablers in Moscow and Tehran, all of Europe and, eventually, the U.S. will be under threat.

After receiving Russian military aid that enabled him to escape near certain defeat by the armed opposition to his regime earlier this year, Mr. Assad apparently is in no mood to talk peace despite the shaky cease-fire between his government and some rebel factions reached last month. Meanwhile, Russian President Vladimir Putin has shown no inclination to push for a political resolution of the conflict that would involve easing Mr. Assad out, nor has Iran's Supreme Leader Ayatollah Ali Khamenei. But it's a virtual certainty that the war won't end as long as Mr. Assad remains in power.

Analysts don't expect Mr. Assad will ever be able to retake the whole of his country, which appears to have been permanently fractured along ethnic and sectarian lines. But he doesn't need to. It's enough for him to maintain control over the territory he now holds, and with a bit of strategic help from Russia he could continue to rule over a rump state carved out of his former country indefinitely. He has no incentive to step down no matter how much his ceding of the rest of Syria to ISIS control threatens Europe.

Likewise, Mr. Putin has gotten everything he wanted from his quick and relatively painless military incursion into Syria. It has not become the “quagmire” for Russia that President Barack Obama predicted it would be, and it ensured Russia's continued access to a major naval base there. Mr. Putin's power play has boosted him domestically, and internationally it has forced the U.S. and its allies to recognize him as an important arbiter of the Syrian conflict and an indispensable partner in resolving the crisis there.

Europe won't begin to be safer until Mr. Assad, who presided over the collapse of his country into a failed state, is gone. But he will never leave as long as he has Mr. Putin's backing, and as a result Europe's terrorist problem depends on the Russian leader's whim. Mr. Putin wants nothing more than to sow disunity among the member states of the European Union, and the refugee crisis created by Mr. Assad gave him a perfect opportunity. Now the terrorist threat from ISIS has given him another.

The vast majority of refugees entering Europe aren't terrorists — and for that matter, most of the suspects in the attacks in France and Belgium were actually citizens of those countries, homegrown terrorists radicalized by ISIS propaganda into believing the group's grandiose vision of an Islamic caliphate uniting Muslims around the world. The success of ISIS in holding territory in Syria and Iraq is crucial to that narrative, and Mr. Putin's actions are enabling it.