Rumsfeld, speaking to Greta Van Susteren on Fox News, pointed out that instead of reaching out and supporting Karzai, members of the Obama administration - including Vice President Joe Biden and Secretary of State Hillary Clinton -- went out of their way to "trash" him.
"I really think it's understandable, given the terrible, terrible diplomacy that the United States has conducted with Afghanistan over the last several years," Rumsfeld said.
Rumsfeld explained that the Bush administration had a great relationship with Karzai, but that since the Obama team came in, "it has gone downhill like a toboggan."
Rumsfeld also criticized the Obama administration for failing to get a status of forces agreement with Afghanistan that would allow U.S. troops to remain past the end of the year.
"A trained ape could get a status of forces agreement. It does not take a genius. We have so mismanaged that relationship," he said.