By contrast, the favored American missile-defense radar operates at frequencies 70 times higher. The advantage of this is that you get 70 times better radar resolution, and thus a far clearer picture of the object. Clarity is important when it comes to figuring out exactly what sort of a missile you are dealing with and distinguishing an actual warhead from debris. However, at such high frequencies the amount of radio energy that is reflected back to the radar is very small, typically one one-hundredth or less of that received when using the lower frequencies of the Russian radar.

What all this means is that the Russian radar can quickly and effectively search the sky for missiles, but has little ability to determine exactly what it has found. The American radar may take longer to find the object, but can carefully observe its structural details. It should be obvious that when you use two systems with such different strengths and weaknesses in tandem, you will have a much easier time spotting and tracking missiles.

Working together at Azerbaijan, which borders northwestern Iran, has another advantage. At such short range, the curvature of the Earth has only a small effect on the radar’s line of sight, so we’d get a clearer view and much earlier warning. American radar there could observe the launching of a missile out of Iran and headed toward Washington at least three to four minutes earlier than could the proposed Eastern Europe system. During these precious extra minutes, tracking data can be accumulated, intercept points calculated, and interceptor missiles launched.

There can be few, if any, technical objections to such cooperation. Politics, however, is another story. Those who do not believe that the cold war is over will complain that we cannot trust the Russians to work with us even when it is in our common interests. Another objection — that President Putin’s government is hardly a paragon of democracy and human rights — ignores the fact that technical cooperation between the countries is a good way to encourage Russia to be closer with the West.

President Bush told Mr. Putin last month that “the cold war is over.” Cooperating with Russia on missile defense is the perfect way to put those words into action.