I know Americans are war-weary and eager to focus on our domestic and economic problems, not foreign affairs. I also know the situation in Syria is complex and there are no ideal options. But the basic choice we face is not complicated: Do the costs of inaction outweigh the costs of action? The events of the past 26 months have convinced me that they do. All of the terrible consequences those against intervening predicted would happen in Syria if we intervened happened because we did not. This conflict will grind on with all of its worsening effects until the military balance of power shifts more decisively against Assad and his Iranian, Russian and Hizballah backers.

The U.S. does not have to act alone, put boots on the ground or destroy every Syrian air-defense system to make a difference. We could train and arm well-vetted Syrian opposition forces, as recommended last year by President Obama’s national-security team. We could strike Assad’s aircraft and Scud-missile launchers. We could destroy artillery and drive Assad’s forces from their posts. We could station Patriot-missile batteries just outside Syria to create safe zones across the border.