U.S. Air Force Chief of Staff General Mark Welsh on Tuesday warned that short-term moves to revise downward the Air Force’s planned purchase of 1,763 Lockheed Martin Corp F-35 stealth fighters could damage the program and scare off foreign buyers. “Let’s delay this discussion for a little while until we see what happens in the world,” Welsh told reporters at the annual Air Force Association conference, citing efforts by Russia and China to field more advanced fighter aircraft of their own.