In the Anbar document, the author describes an al-Qaida in crisis, with citizens growing weary of militants' presence and foreign fighters too eager to participate in suicide missions rather than continuing to fight, said Rear Adm. Gregory Smith, a U.S. military spokesman.
"We lost cities and afterward, villages ... We find ourselves in a wasteland desert," Smith quoted the document as saying.
The memo cites militants' increasing difficulty in moving around and transporting weapons and suicide belts because of better equipped Iraqi police and more watchful citizens, Smith said.
The author of the diary seized near Balad wrote that he was once in charge of 600 fighters, but only 20 were left "after the tribes changed course" a reference to how many Sunni tribesmen have switched sides to fight alongside the Americans, Smith said.
The switch by the Sunni tribes, whose resulting U.S.-backed groups are often referred to as awakening councils, has been credited with helping reduce violence across the country.
The councils were key to helping push al-Qaida out of Anbar province, once one of the country's most violent. The terror group's top leaders are now based somewhere in northern Iraq, Smith said, having moved out of Anbar and into Diyala province last year.
The U.S. military described both documents, but allowed reporters to see just four pages from them, citing security reasons.
The documents tell "narrow but compelling stories of the challenges al-Qaida in Iraq is facing," Smith told reporters in Baghdad's heavily guarded Green Zone.
"This does not signal the end of al-Qaida in Iraq, but it is a contemporary account of the challenges posed to terrorists from the people of Iraq," Smith said.
He said the documents are believed to be authentic, Smith said, because they contain details that only al-Qaida in Iraq leaders could know about battlefield movements and tactics.