Gunmen slipped into a college campus under cover of fog Wednesday, killing at least 20 people — some of them shot execution-style — in the latest terrorist attack in Pakistan targeting students in apparent revenge for expanding military crackdowns. The attack in Charsadda, about 30 miles from Peshawar, was claimed by a Taliban faction. It is likely to unite the country behind stern action against Islamist militants 13 months after a similar rampage at a nearby army-run school killed about 150 students and teachers.