Rumsfeld's comments came during an interview published Monday by the German news magazine Der Spiegel.
"I think moving away from his proposed military action could be a good thing," the magazine quoted Rumsfeld as saying. "President Obama accepted the [Russian President Vladimir] Putin proposal because there were no good options left. I suspect the president wishes he had not gotten to this point. I think he might have been better off if he had helped some of the non-radical elements among the Syrian rebels early on, not with U.S. troops, but with weapons, intelligence and humanitarian support."
"Without doing that, the factions that seem to be prevailing, at least to this date, are the ones that are the best organized, the most disciplined, the toughest, and the best financed -- some of which are radical Islamists, even though the Islamists most likely do not represent a majority in that country."
Rumsfeld, who served as defense secretary under Republican Presidents Gerald Ford and George W. Bush, also said he believes Obama, a Democrat, should be wary of Putin and his proposal for Russian supervision of Syria's chemical weapons.
"We don't yet know the details or who would do the supervising, but my recollection is that the Soviets or the Russians gave Syria most of their weapons arsenal," Rumsfeld said. "And it is not clear to me that Russia itself has fully complied with all their obligations with respect to chemical weapons. Therefore, a question could be: Might it possibly amount to putting the fox in charge of the chicken coop?"