In their three major theaters of war, Islamic State terrorists appear to have been knocked back on their heels. That undoubtedly is good news, but it comes with a worrisome qualification.

Iraqi government forces are making slow but steady progress in ousting IS troops from the important city of Fallujah.

Syrian offensives, backed by Russia and Iran, have pushed the IS back. The terrorist group may be on the verge of losing its unofficial capital in Raqqa province.

And in Libya, government troops have entered the IS center of strength in the coastal city of Sirte.

Obviously, an enormous amount of blood will have to be shed before the IS can be eradicated as a powerful organization. But even that might not mean the end of IS terrorist attacks.

Though IS leaders have ordered massacres in France, Belgium and the United States, their primary focus for many months appears to have been on conventional military victories. They have had plenty to celebrate.

Armies facing each other on a battlefield are not terrorists’ natural, instinctive strategies, however. Vicious brutality directed against innocent civilians who cannot fight back is what they prefer.

Now, losing in conventional warfare, the IS leaders may revert to form and launch even more terrorist attacks. U.S. Homeland Security officials should be planning for that scenario.

Big-game hunters are well aware that powerful animals such as tigers can be most dangerous when they are cornered. That may be the case with IS terrorists, too.