WASHINGTON — The Pentagon’s failure to detect the extramarital affair and “swinger lifestyle” of a top general despite background checks that likely included polygraph tests exposes flaws in vetting those entrusted with the nation’s most sensitive national security secrets, according to experts and a top senator. The Army fired Maj. Gen. David Haight from his job overseeing operations at European Command earlier this year after investigators uncovered his double life. Haight’s post required him to have access to the military’s classified capabilities to thwart Russian aggression, and his double life would have put him at risk of being blackmailed, several senior officials have said. But Haight’s dark secret came to light only after the military received anonymous tips about his 11-year extramarital affair and other sexual escapades and began investigating in December. Officials entrusted with the most sensitive information undergo background checks and lie-detec...