Despite a solemn promise made last fall, Russia and Syria have started a massive bombing campaign against a rebel Syrian province, targeting civilians with barrel bombs, so far displacing an estimated 200,000 people. The United States and world opinion must apply maximum pressure to halt a potential slaughter of innocent people.

Last fall, as Russia and Syria planned a major offensive to drive a small number of rebels from Idlib province that was expected to endanger huge numbers of refugees from other war-torn areas of Syria, President Donald Trump spoke up forcefully, saying the two “must not recklessly attack” the civilian areas. He must do so again.

The massive attack on Idlib did not materialize due to an agreement between Turkey, which assumed a protective role for the endangered civilians, and Russia to create a demilitarized zone for the roughly 3.5 million civilians, including omore than a million refugees, living in the province.

Following the agreement, Trump claimed credit for stopping the planned assault, apparently because of pressure the United States quietly put on Russia.

Now the threat is back. It brings the potential for more human misery and new floods of refugees fleeing Syria for Turkey and Europe.

In the recent attacks, Syria is once again using barrel bombs dropped by helicopters and designed to wound and kill civilians. The United Nations Security Council in 2014 specifically condemned the use of these cruel and devastating weapons, and Russia’s ambassador to the UN said in 2015 that Syria would no longer use them, a promise that Syria and Russia have ignored. Their use in the Syrian civil war is considered a war crime.

On May 3, reported The Wall Street Journal, Syrian helicopters dropped more than 100 barrel bombs on villages and hospitals in what was ominously described as a prelude to a ground offensive.

The independent Syrian Network for Human Rights, which has carefully documented the use of these weapons during the civil war, reported that Russian and Syrian bombardments have killed 441 civilians including 130 children since last September’s ceasefire. The recent bombings, which continued last week, have killed an estimated additional 100 civilians and damaged 10 hospitals. Unfortunately, this might be just the beginning of broader, unconscionable carnage.

The attacks appear to signal the end of Russian-Turkish cooperation to preserve a ceasefire in the province. As recently as April 8, Turkey’s President Recep Tayyip Erdogan and Russia’s President Vladimir Putin agreed on a plan for joint Russian-Turkish patrols to prevent attacks by rebel forces in Idlib. “Another significant step taken with Russia is to cooperate against terrorist acts in Syria,” said Erdogan at the time. It turned out to be another broken promise.

In a statement issued recently, State Department spokesman Morgan Ortagus said, “We call on all parties, including Russia and the Syrian regime, to abide by their commitments to avoid large-scale military offensives, return to a de-escalation of violence in the area, and allow for unhindered humanitarian access to address the humanitarian disaster created by the ongoing violence.” The European Union has also protested.

These are fine words, but unless backed by a credible threat of consequences, they are unlikely to have any impact.

Meanwhile, other reports say Iran has made preparations to attack U.S. forces in Eastern Syria.

Trump is being tested. To prevent a massive loss of civilian life, the president needs to make it clear to Russia, Syria and Iran that they must back off.