AMERICAN envoy Richard Holbrooke is being called the ''Henry Kissinger of the Balkans.'' And the Clinton administration cautiously is beginning to consider that Bosnia - of all things - may become an election-year asset. An American policy that relies heavily on brute military force to bring peace to Bosnia, and not the nuanced diplomacy of West Europeans, is working so far. Croatian Army tanks - initially with Washington's tacit approval - and NATO jets have pummeled the Bosnian Serbs. Gains on the ground have created a situation close to the roughly 50-50 partition of Bosnia that negotiators fruitlessly have been seeking for two years. But as the US warily eyes a Croatian Army advance on Serb-held Banja Luka, whether Croatia will be a foreign-policy friend or Frankenstein is becoming crucial. Despite a public pledge by Croatian Foreign Minister Mate Granic that the Croatian Army would halt its offensive, Croatian troops continued to cross the border into Bosnia 40 miles north of Banja Luka. A Croatian Army that once appeared to be working as American proxies, allowing the politically unpalatable option of US ground troops in Bosnia to be avoided, is now something worried US officials are trying to control. ''It's like the Iran-Iraq war, when the Americans were so obsessed with stopping Iran that they didn't think about who the Iraqis were,'' says a Tuzla-based United Nations official. ''Then they realized the Iraqis were just as crazy as the Iranians. Here, the Croats are just as crazy as the Serbs.'' UN officials warn that a Croatian attack on Banja Luka could prompt Serbia's President Slobodan Milosevic to send his Army into Bosnia to defend the city. Mr. Milosevic faces a threat to his political future if Banja Luka falls and some 300,000 Serbs flee to Serbia proper. A Croatian attack could quickly lead to the collapse of the American peace initiative and widen the war. It also could prompt a UN pullout from Bosnia assisted by American ground forces. The demand by Croatian President Franjo Tudjman that Serbia give up control of Eastern Slavonia, an oil-rich strip of Croatia seized by rebel Serbs in 1991, also threatens to scuttle the talks. Whether President Clinton will be able to boast that American leadership brought peace to Bosnia now lies largely in the hands of one man: President Tudjman. In a six-month period, Tudjman has gone from being seen as the clumsiest of the leaders of Serbia, Bosnia, and Croatia to the shrewdest. The former Yugoslav Army general used his rearmed and retrained Croatian Army last month to eject 150,000 rebel Serbs from almost all of the 30 percent of Croatia they seized four years ago. The stunning gains of allied Bosnian and Croatian forces over the last week are based on the arrival of Croatian Army tanks and artillery in Bosnia - and not simply NATO bombing. Only the Croatian Army, not the poorly equipped Bosnian Army, is believed to have the firepower to take Banja Luka. The Clinton administration, led by Ambassador to Croatia Peter Galbraith, has vigorously pursued policies that allowed Croatia's economy to boom and fund a billion-dollar Croatian arms-buying spree. A retired US general, who says he has no links to the American government, is working as an adviser to Croatia and has helped it shift from Warsaw-Pact-style military tactics to more-effective NATO-style tactics. The United States has also turned a blind eye to Croatian violations of a UN arms embargo imposed against all of the former Yugoslavia. Tudjman is eager for Croatia to join the European Union. Just how aggressive he is going to be in Bosnia remains unclear. His troops burned thousands of Serb homes and killed dozens of Serb civilians during last month's offensive. His loyalty to his Bosnian Muslim allies is not considered strong. He has long opposed the development of a strong Bosnian government Army and has prevented shipments of tanks and heavy artillery from crossing Croatia and entering landlocked Muslim-held territories. The capture last week of more than 1,500 square miles of Serb-held territory should have allowed the Bihac-based Bosnian Army Fifth Corps to link up with Muslim-led forces in central Bosnia. But no link has occurred. Croat Army and Bosnian Croat forces lie between. ''The Bosnians are desperate to link up their forces,'' a Tuzla-based European diplomat says. ''Right now, they are paying huge fees for any goods that cross Croat territory.'' In a joint statement issued after a meeting with Mr. Holbrooke in Split, Croatia, Tudjman and Bosnian President Alija Izetbegovic said that just how the newly captured territory would be divided between the Muslims and Croats would be decided peacefully. But western officials worry that Tudjman may see gaining control of Banja Luka and all of western Bosnia as simply too tempting. ''It makes sense for Croatia to take Banja Luka,'' the European diplomat says. ''They'll have a direct line across Bosnia.''