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Washington • Former Defense Secretary Robert Gates defended his criticism of President Barack Obama's handling of the Afghanistan war while the president is in office and troops are still fighting, saying he thought it important to be candid in his new memoir.

"I don't think that waiting until 2017 to weigh in on these issues, and in a comprehensive and thoughtful way, made any sense," he told CBS's "Sunday Morning."

In his book, titled "Duty," Gates doesn't take issue with Obama's big policy decisions but accuses him of an "absence of passion" and an unwillingness to make it clear to the troops that their success was critical to U.S. national security. He also writes that Obama became convinced that his war strategy would fail within a few months of committing to it and seemed to have grave doubts about it from the outset.

"People gave me a lot of credit when I was in office of being blunt and candid about what I felt about things," Gates said in the interview. "I could hardly be any less in writing a book."

Excerpts from the 600-page-plus memoir, which goes on sale Tuesday, quickly become fodder for political debates. Both Republicans and Democrats worried that Gates's critique of a sitting president and his administration would sew distrust within future administrations and chill internal debates.

Such concerns, however, didn't stop lawmakers from using the book's passages to score political points. On CBS's "Face the Nation," Sen. Marco Rubio, R-Fla., said Sunday that the book showed that the president and his top aides had "the wrong convictions" or "lack the courage" of their convictions.

Sen. Benjamin Cardin, D-Md., cited passages from the book, which critics have called either nuanced or contradictory, to defend the president. Gates gives "high praise to Obama," Cardin said on NBC's "Meet the Press." He added, "Just about every major decision Obama made, Gates agreed with."

Gates seemed perplexed with Washington's reaction to his memoir, suggesting that the controversy surrounding his book was a sign of the sad state of affairs in U.S. politics.