The nation’s top Marine said Taliban attacks had grown “more desperate” as they looked to inflict casualties on American troops drawing down from Afghanistan.
"As we come down in size, there are [fewer] opportunities to interdict us, and they become more desperate," said Marine Corps Commandant Gen. James Amos, speaking at the American Enterprise Institute.
"It took the wind out of us," said the commandant, adding that it had been at least five months since the Marines last suffered a casualty. "It was a rude awakening."
"There's no question that the Taliban will continue to probe and look for opportunity," he added.
Amos said there are currently about 4,000 Marines in Helmand and Nimroz provinces in Afghanistan, down from about 7,000 last year.
The U.S. drawdown from Afghanistan is raising fears that local security forces are unprepared and could suffer the same fate as Iraq’s military, which has lost control of major cities to an al Qaeda-inspired group, the Islamic State in Iraq and Syria.
President Obama last week announced 300 military advisers were headed back to Iraq to assist local forces.
The U.S. sought to leave a force of several thousand in Iraq after the war, but failed to reach a security agreement with Baghdad to grant those forces immunity.
The White House is planning to leave a residual force of 9,800 U.S. troops in Afghanistan after their combat mission ends in December to continue training and advising Afghan forces, and conduct counterterrorism missions.
However, that presence will halve by the end of the 2015, and drawn down to a small force to protect the embassy by the end of 2016.
Amos said it has not yet been decided how many Marines would make up the 9,800 U.S. force, but that the "clear bulk" would be gone.
He added that the security meltdown in Iraq was heartbreaking, but Marines should feel proud of their accomplishments there.
"Does it break my heart? You bet," he said.
"When we left ... the governance was there ... the streets were clean. ... We completed our mission. We should all feel good about that, regardless of how it turned out," Amos said. "That ground was sanctified. You can't take that away from us."
Amos said he was more optimistic about Afghanistan's future.
"The Afghan national army forces ... they're trained and battle-hardened," he said.
"I think we've done what we've set out to do," Amos said.
"We are prepared for the first time to have a democratic turnover,” he said of Afghan elections. “I feel pretty good about it."