The expulsion came as the US and Russia clashed over Kosovo, and Moscow made its anger clear by sending bombers into American airspace. After a decade of warming relations, the cold war is starting to chill once again.
The American embassy said that Lieutenant-Colonel Peter Hoffman, an assistant army attache, left Russia on Thursday after his accreditation was withdrawn for unspecified reasons. Defence attaches' posts are known to be used sometimes as cover for intelligence work. Asked if the expulsion was related to disagreements over Kosovo, the White House said: "I would steer you away from that." But tension has been growing between Russia and America over Kosovo since Nato asked countries that border Serbia to deny permission for Russia to fly its troops into the country.
To underline Russia's assertiveness, a week ago Russian Tu-95 Bear bombers flew into the airspace of Iceland, a Nato member. US fighters intercepted them and escorted them away. Elsewhere, Russian Tu-140 Blackjack bombers flew down the Norwegian coast.
The United States has been trying to calm the reports of a growing split between Washington and Moscow, but privately British and American officials have always feared that a price would be paid for Nato's unwillingness to consult Russia over the war in Kosovo.
British diplomats have been reporting growing anger in Moscow and warning that there would be a backlash. When Russian troops suddenly appeared at Pristina airport last month, it triggered near-panic in Washington.
The US Defense Secretary, William Cohen, said: "They want to be seen as remaining a force that has to be dealt with." One factor inhibiting Nato from even talking about a deployment of ground troops in Kosovo during the conflict was a desire to avoid antagonising Russia even further.
The overflights were part of "West '99", Russia's biggest military exercise for years.
"The exercises were planned in December 1998," said Colonel-General Yuri Baluyevsky, first deputy chief of the Russian general staff, last week. But "events in Yugoslavia were certainly taken into account in planning the exercise", he acknowledged.Reuse content