What keeps Pentagon generals up at night?

In a word: China.

Air Force Lt. Gen. Herbert J. Carlisle, deputy chief of staff for operations, said China’s rollout earlier this year of a new J-20 stealth fighter, which has made two or three test flights, is very troubling, along with another joint Russian-Indian stealth jet.

Both aircraft could be sold to Iran and affect a future U.S. intervention there against Tehran’s nuclear program.

“Those are discouraging in that they rolled out in a time that we thought there was maybe a little bit more time, although we weren’t sure of that,” Gen. Carlisle said.

The three-star general’s comments echoed earlier comments by Navy Vice Adm. David J. Dorsett, a senior intelligence official, who said of the J-20 in January that “we have been pretty consistent in underestimating the delivery of Chinese technology and weapons systems.”

U.S. military fighters will remain a pace ahead technologically of both the Chinese and Russian stealth jets. But if there are further F-35 delays, “then that pacing is in jeopardy,” Gen. Carlisle said.

For a generation, or really since the Korean War, the US has been able to count on a vast technological superiority gap to make up for the one advantage China could bring to bear: Sheer numbers. The technological gap is shrinking, and a time when the US economic advantage is also disappearing and when China has become bolder in supporting rogue states.