U.S. military commanders have said in recent months that they feared such an increase in violence would accompany the planned withdrawal of most American troops by the end of the year. Military officials in Baghdad and at the Pentagon blamed the mounting death toll on the growing sophistication of weapons that insurgents and Iranian-backed militia groups are using, including powerful rockets, armor-piercing grenades and jam-resistant roadside bombs suspected of coming from Iran…

During much of the insurgency that followed the 2003 U.S.-led invasion, U.S. military commanders blamed Sunni-dominated terrorist groups such as al-Qaeda in Iraq for many of the attacks against American troops. But as the U.S. military has adjusted its tactics, largely withdrawing from cities and improving its technological capacity to combat deadly roadside bombs and suicide attacks, officials say it has become far harder for loosely organized Sunni militias to strike out against the roughly 46,000 U.S. troops in the country.

Now, according to Maj. Gen. Jeffrey S. Buchanan, chief spokesman for the U.S. military in Iraq, the primary threat to the Americans comes from three Shiite militia groups operating in Iraq, which officials said they believe are being trained and equipped by Iranian Revolutionary Guard Corps special forces.