IN THE FIRST GULF war, its score card against Russian-built Iraqi tanks was, approximately measured, 1,245 to zero. But like a great mythic warrior, the Abrams has an Achilles' heel. It can be killed from behind by a well-placed antitank missile aimed at a small chink in its armor.
        So far in Operation Iraqi Freedom, the American military has lost two Abrams tanks. The first M1s ever destroyed by enemy fire in battle, they were caught in an ambush of the U.S. Army's 3/7 Cavalry near As Samawah, on the west bank of the Euphrates River. Two is not a large number, and the Coalition forces have at least 650 tanks in Iraq with more on the way. But U.S. officials are worried about the skill or at least the fanaticism of the guerrilla fighters who sneaked up on the tanks driving a "technical," a jeep, under cover of a sandstorm. More worrisome are the type and the source of the weapon apparently employed, a Russian-made Kornet antitank missile.