As NATO’s war against Yugoslavia enters its fifth week, it is time to acknowledge a few strategic realities. NATO’s war aims have expanded dramatically, with the alliance now committed to the safe return of Kosovar refugees and the withdrawal of all Serb forces. Ethnic cleansing can be reversed, but air power alone is ill-suited to this task.

NATO must defeat Slobodan Milosevic’s strategy, and for that, some form of ground force will almost assuredly be required. The question is: what kind? Will it be a U.S.-European force to liberate Kosovo, indigenous Kosovars who spearhead their own liberation or a combination of both?

NATO can secure its aims any number of ways, but each of these options will require from three to twelve months to achieve its intended effect. Key to success, therefore, is the perseverance of the alliance.

While not the disaster that many are alleging, the allied air campaign in Yugoslavia is unlikely, by itself, to lead to Milosevic’s defeat. First, allied air operations will continue to be hampered by limited visibility. Relatively clear skies are required to use the laser-guided bombs that make up the bulk of NATO’s direct attack capability, yet such favorable conditions exist only 25 % of the time over Yugoslavia during spring.

Second, Serb forces in Kosovo have dispersed and are occupying Kosovar dwellings that have been vacated as a result of ethnic cleansing, which significantly reduces their vulnerability to air attack. Third, Serb forces are using civilian vehicles to move about Kosovo and using captive Kosovars as human shields, which limits their vulnerability further still. Fourth, cutting off supplies to an infantry-based Serb force that has had months to preposition caches will produce little near-term effect.

There are two basic ground force options that could achieve NATO’s new war aims. The first, direct intervention by NATO ground forces, has much greater certainty of success and could be accomplished in much less time, but at the risk of NATO casualties that could easily number in the hundreds. The second, raising and supporting a Kosovar force, would require substantially more time and would be less certain to succeed. If NATO governments continue to oppose the use of allied ground forces, however, arming the Kosovars will be the best option available.

Whereas a heavy armored force of 200,000 soldiers is probably the minimum number NATO would need to overthrow Slobodan Milosevic, a force of about 80,000 should suffice to liberate Kosovo, with about 60 % coming from the U.S. This size force would give NATO a 2:1 advantage over Serb forces in the province. Since the principal problem in Kosovo is dealing with the dispersed Serb paramilitaries, light, mobile NATO forces -- helicopter assault and mountain troops -- could bear the brunt of the fighting. Getting them there and ready will take two-to-three months, and ejecting the Serbs could take that much time again.

(The idea of the Europeans taking on the burden of liberating Kosovo by themselves is a non-starter. European militaries mustered only 50,000 of the more than 500,000 in Operation Desert Storm, and currently have 20,000 troops tied down in Bosnia. They are also far less capable than their U.S. counterparts.)

Alternatively, NATO could recruit, train, and equip a Kosovar ground force. If ever there was a guerrilla war that was winnable, this is it. Developing an indigenous force to wrest control of Kosovo from the Serbs will take several months, but such a force would benefit from highly favorable conditions for irregular warfare:

• A friendly external sanctuary in Albania

• Mountainous terrain in which to operate

• A radicalized population within and outside Kosovo that provides fertile recruiting ground for freedom fighters

• Access to the world’s most formidable air force and sophisticated intelligence apparatus -- ours

Admittedly, there are significant risks in the Kosovar alternative. It will take the longest time, and carries the least certainty of success. Rather than simply reversing Serb ethnic cleansing, Kosovars are likely to engage in some of their own.

But NATO also has considerable leverage over the Kosovars. For example, a NATO-supported indigenous Kosovar army will be critically dependent on allied air cover. Additionally, as NATO builds a new force from among the refugees, it can choose leaders of its liking. The Kosovars are going to fight whether NATO arms them or not. NATO support would give the alliance at least some control over the outcome.

A hybrid force of, say, 60,000 NATO troops and 20,000 Kosovars may provide the best of both options. The critical requirement for any of these to work is time. A key question is, on whose side does time lie? It lies on NATO’s, that is, if the alliance now acknowledges that ground forces are necessary.