FOR MONTHS, it has been evident to many global observers that a military intervention would be necessary in Mali, where Islamist radicals seized the northern half of the country last April. But planning for such an operation by African nations was prolonged, and though the U.N. Security Council approved an intervention, its deployment was not envisioned until this autumn. On Friday, French President Francois Hollande sent troops to Mali to confront the radicals, a decisive break in the international strategy and one that signals difficult choices to come.

Mr. Hollande’s action came in response to an assault by the Islamist fighters on Konna, a town about 375 miles northeast of the capital, Bamako. When the radicals marched into Konna, Mali’s weak military fled.

In April, Islamist radical groups linked to al-Qaeda in the Islamic Maghreb captured the huge, arid north and imposed a Taliban-style rule that was so brutal — featuring amputations as punishment — that hundreds of thousands of people have run for their lives. The fighters destroyed ancient landmarks in the north and now control an area larger than France.

News of the capture of Konna unleashed panic. The president of Mali, Dioncounda Traore, sent an appeal for help to the Security Council and to France that was described by Susan Rice, U.S. ambassador to the United Nations, as saying, in effect, “Help, France!” Within hours, Paris reacted with the first contingent of Western troops to be sent to Mali. The mission appears to be to stop the advance of Islamist forces. “This operation will last as long as needed,” Mr. Hollande vowed.

Whether this will block the militants is not clear. But the events of recent days have demolished the leisurely timetable set by the Security Council. The march on Konna seems also to have dampened any hopes for peace talks between the government, the Islamist groups and Tuareg separatists, set to begin Jan. 21 in neighboring Burkina Faso.

In fact, a non-military solution seems unlikely. Mali already is split in half, and the Islamist groups have dug into their strongholds in Timbuktu and Goa. They will be dislodged only by force. The northern desert makes any military option exceedingly difficult, so a counteroffensive will need to focus initially on the cities the radicals control. It may also require a change of heart by Algeria, a key power in the region that ruled out contributing to an African security force.

For now, Mr. Hollande’s military move is a welcome expression of determination. But it can hardly be more than a stopgap measure, insufficient to reverse the occupation of the north. Mali must be rescued from becoming a failed state and a haven for the Islamic radicals.