Both men fielded pointed questions about the viability of the Obama strategy, the reliability of the Afghan government and the wisdom of announcing the July 2011 withdrawal start.
"They were not credible in power, and they are not credible now," McChrystal said.
Eikenberry, who previously had privately expressed doubts about sending a large number of additional US troops to Afghanistan, said he was now "100 percent" behind the strategy, which includes emphasis on a bigger role for US civilian agencies to assist in strengthening the Afghan government and its economy.
John McCain, the senior Republican on the Senate Armed Services Committee, praised the decision to send more troops but said that setting a firm date for beginning to send them home was a mistake.
"We have announced a date divorced from conditions on the ground," McCain said.
Eikenberry, himself a former US commander in Kabul, said the course outlined last week by Obama "offers the best path to stabilize Afghanistan and to ensure al-Qaida and other terrorist groups cannot regain a foothold to plan new attacks against our country or our allies."
McChrystal spoke not of defeating the Taliban but of attempting to "disrupt and degrade" its fighting capacity. "Rolling back the Taliban," he said, "is a prerequisite to the ultimate defeat of al-Qaida."
The House panel's highest-ranking Republican, Howard "Buck" McKeon of California, told McChrystal he was waiting to hear how "the president is not under-resourcing his own strategy," since the general had sketched ways that as many as 80,000 additional US forces could have helped turn the tide.
McChrystal said he did not think he would need to ask for any more troops in a year's time but would not hesitate to recommend more if circumstances changed.
He also told McKeon he did not recommend the July 2011 exit plan but supports it.
"By the summer of 2011, it will be clear to the Afghan people that the insurgency will not win, giving them the chance to side with their government," McChrystal said. "From that point forward, while we begin to reduce US combat force, levels, we will remain partnered with the Afghan security forces in a supporting role to consolidate and solidify their gains."
"Afghans do not regard us as occupiers," he said. "They do not wish for us to remain forever, yet they see our support as a necessary bridge to future security and stability."