A Syrian youth standing next to the rubble of a destroyed building in the besieged Eastern Ghouta region on the outskirts of Damascus on Monday. (Abdulmonam Eassa/AFP/Getty Images)

ONCE AGAIN the Syrian regime of Bashar al-Assad is conducting a brutal and criminal offensive against its own population, with the support of Russia and Iran. Warplanes have been pounding the suburban Damascus area known as Eastern Ghouta, targeting hospitals, apartment buildings and other civilian sites. In the week that ended last Sunday, relief organizations reported at least 541 people killed and 1,500 wounded. On Sunday, hours after the U.N. Security Council passed a resolution mandating a 30-day cease-fire, the offensive intensified: Ground forces launched an assault on five fronts, and opposition sources reported that chlorine gas had been used in at least one area.

This latest Syrian atrocity has been made possible, like so many before it, by Vladi­mir Putin. The Russian military is backing the Ghouta offensive, and Russian diplomats ensured that the Security Council resolution meant to stop it was held up for several days, then laced with loopholes providing a pretext for the slaughter to continue. On Monday, Mr. Putin offered, instead of the cease-fire, a daily “humanitarian pause” to allow the evacuation of civilians and entry of aid. Moscow said it would begin on Tuesday, but — to the surprise of virtually no one — no such action was taken. Instead, the assault goes on.

Syria has become a maelstrom of war that has sucked in half a dozen outside powers, including the United States, which has some 2,000 troops deployed in the country. But most of the conflict is waged, supported or manipulated by Mr. Putin, who aspires to use Syria to reestablish Russia as a Mideast power at the expense of the United States. In addition to aiding and abetting the scorched-earth campaigns of the Assad regime, the Kremlin appears to have signed off on a Feb. 7 attack by irregular Russian forces on U.S. and allied positions near the Euphrates River in eastern Syria. The assault was rebuffed with heavy Russian losses, but it showed Moscow’s audacity in risking a direct U.S.-Russian conflict.

Russia has suffered several recent reverses in Syria: not just the bloody nose on the Euphrates but the collapse of a unilateral attempt to broker a peace settlement outside the long-standing U.N. diplomatic process. Yet Mr. Putin does not appear chastened. Now he is openly defying the Security Council while helping the Assad regime to overrun a region populated by 390,000 people by bombing hospitals and deploying chemical weapons. He shows no interest in the U.N. negotiations, which call for the formation of a new Syrian government acceptable to all sides, followed by free elections.

After months of hesi­ta­tion, the Trump administration recently outlined a policy for Syria that supports the U.N. process and calls for eliminating terrorist groups; officials say U.S. troops will remain in the country, which provides Washington with some diplomatic leverage. But Mr. Putin eschews cooperation with Washington. Instead, he is doing his best to bluff and intimidate President Trump into ordering a withdrawal. In the absence of a firm U.S. response to its latest outrages — and so far there is no sign of one — the Kremlin is unlikely to change course.