We shouldn’t have to do it alone. A strong international coalition is needed to halt the Islamic State threat that has gained momentum in Iraq and Syria. Given the number of Europeans and Americans who have joined the Islamic State and adopted its violent tactics, and who have the freedom to travel to and from the region, the United States cannot assume it is immune from this threat. President Obama deserves credit for his leadership in mobilizing countries throughout the world to quash these militants. But more must be done to get allies to take an active role in the fight and to slow the movement of would-be jihadists traveling to the region.