The two U.S. Black Hawk helicopters from the 101st Airborne Division destroyed Saturday in Mosul brings the total of helicopters downed in hostile actions in the past two weeks to four, with 39 service members killed and 25 wounded.

Saturday’s crashes also confirm an ominous new focus in attacks by insurgent forces in Iraq: From May 1, when President Bush declared an end to major combat operations, to Oct. 25, there were eight helicopter-related deaths, but these were the result of accidents, not hostile fire.

Then, on Oct. 25, anti-U.S. forces used a rocket-propelled grenade to bring down a Black Hawk in Tikrit. The RPG hit one of the helicopter’s engines, forcing it to make an emergency landing in a field. The five crew members came under intense small-arms fire but managed to escape before the Black Hawk was engulfed in flames.

On Nov. 2, one of the deadliest days of attacks on U.S. troops since the occupation began, guerrillas shot down a U.S. Chinook helicopter carrying dozens of soldiers near the city of Fallujah west of Baghdad, killing 16 soldiers and injuring 20 others.

Five days later, another Black Hawk was shot down on Nov. 7 near the insurgent stronghold of Tikrit, killing all six soldiers on board.

In the first two weeks of this month, helicopter-related losses accounted for about 70 percent of the total U.S. troops killed in hostile action.

The spike in helicopter-related deaths comes just after the 10th anniversary of what came to be known as “Black Hawk Down”: the downing of two such helicopters in Mogadishu, Somalia, on Oct. 3, 1993. That incident, which resulted in the deaths of 18 U.S. soldiers, followed months of operations in which helicopters could move about the area relatively freely.

The U.S. Army is more dependent on helicopters than any other in the world. And within the Army, the 101st Airborne Division is more dependent on helicopters than any major conventional Army unit.

As it turns out, the Black Hawk is more vulnerable than attack helicopters, Army commanders have found, because it generally has fewer defensive counter-measures and also has fewer offensive weapons. The Army has come to prefer OH-58 scout helicopters for most urban missions in Iraq because their small size and nimbleness makes them less vulnerable to ground fire.