How could a gunman get close enough to nearly assassinate a former president?

The attempt to assassinate former President Trump at a campaign rally in Pennsylvania on Saturday raised questions about security protocols and how a gunman could open fire so close to the presumptive GOP nominee.

The shooting, which left a spectator dead and injured Trump and at least two others, stunned operatives of both political parties who have seen firsthand the level of precision, care and detail the Secret Service takes in safeguarding its charges. American presidents and former presidents are among the most protected politicians in the world, with multiple layers of security — some visible to the public and others...