U.S. President Barack Obama is sending Special Forces. British jets have joined French warplanes over the skies of Syria. Even Germany, whose post-World War II constitution puts restrictions on fighting battles on foreign soil, is becoming increasingly involved. But as the West steps up its war against ISIS, it appears that the involvement of the U.S.-led coalition's Arab members -- all of them much closer geographically to the terror group than their Western partners -- is drawing down.

Saudi Arabia and the United Arab Emirates are down to about one mission against ISIS targets each month, a U.S. official told CNN on Monday. Bahrain stopped in the autumn, the official says, and Jordan stopped in August. CNN contacted all of these countries for comment and is yet to receive a response.

Why aren't Arab countries more involved in the fight against ISIS?

Analysts say Yemen is at the center of a proxy war between Saudi Arabia and Iran, the region's biggest powers. Religion and ethnicity are at the heart of the longstanding hostility between the two countries. Iran is majority Shia Muslim and non-Arab. Most of the other countries in the region -- including, and led by, Saudi Arabia -- are majority Sunni Arab, and are suspicious of Iran's motives. So when Iranian-backed rebels seized Sana'a, the Yemeni capital, last year, a Saudi-led coalition of Arab states (including Egypt, Jordan and the UAE) was launched to try to defeat them.

"The critical shift was the coalition in Yemen," says Fawaz Gerges, a professor of Middle Eastern Studies at the London School of Economics. "You're talking about a major 24/7 war. The Saudis and the Emiratis -- the two countries with the most capacity in terms of air power -- are flying fighter jets over the skies of Yemen, so that's why you really have to prioritize the fight in Yemen over the fight against ISIS."