Because Blackbeard, the Shichibukai, has captured Ace, who is Whitebeard's second in Command. Whitebeard sees all of his crewmates as his children, so Whitebeard will attack the government if necessary to save one of his Sons. Also, the World Government sees the execution of Ace as the chance to rid the world of the blood of Gol D. Roger as Ace is Roger's son.