SANGIN, Afghanistan – Sgt. Johnathan Cook’s instructions to his Marines were clear before they pushed Tuesday morning into the notorious “Fish Tank” section of Sangin. “Everyone knows the atmospherics yesterday got a little weird,” he said. “Keep your head on a swivel. We all know the summer offensive is supposed to start in the next 10 days, so expect we could take contact any time.” The ominous directive came before photographer James Lee and I left Patrol Base Fulod with his unit, an element of Baker Company, 1st Battalion, 7th Marines, out of Twentynine Palms, Calif. Second Squad, 3rd Platoon,…