Beyond the fact that the security situation is still tenuous, U.S. soldiers are likely being targeted more now because there is talk that Iraqi and American officials will try to keep additional troops in the country past the December deadline to pull all U.S. forces out, according to Feisal Istrabadi, a former Iraqi diplomat to the United Nations who now teaches law at Indiana University. A coalition of militant groups and outside actors is strongly opposed to that and are using violence to send a message to Washington.

"That's the primary driver," said Michael Knights, a fellow at the Washington Institute for Near East Policy, who tracks Iraqi security issues closely. "The Iranians and Sadrists are taking it very seriously."

The Sadrists are a sectarian militia affiliated with hard-line cleric Moqtada al-Sadr, who adamantly opposes the U.S. presence.

In 2008, the United States and Iraq agreed that all American forces would leave the country by the end of this year. The U.S. is open to keeping troops beyond that date, but only if Iraq asks, according to the Associated Press.

And it's not clear yet that the Iraqis will. Prime Minister Nouri al-Maliki is under significant pressure from political allies, including Sadr, whose backing last year allowed him to win a second term as prime minister.

According to the New York Times, Sadr has said that unless the United States fully withdraws its troops by the end of the year, he will reactivate his Mahdi Army, which was responsible for much of the violence against U.S. troops earlier in the war but was formally disbanded in 2008.

Iran also opposes an extension, said Istrabadi. He said various groups that don't necessarily completely agree with each other are working together. "It's a situation where the enemy of my enemy is my friend."  

Knights said that when talk of an agreement heated up beginning in the spring, attacks on U.S. soldiers and personnel also increased -- including attacks on U.S. bases, with more sophisticated weaponry and an increased quality in the attacks, which Knights said indicates Iranian backing.

"They raised their game, so to speak," Knights said. "They brought in more experienced operators and are supporting Shiite militants in southern Iraq. The result has been better lethality."

The message, Knights said, is "Don't stay. Reconsider."