HAVANA - Cuba's armed forces launched three days of intense military exercises across the island Thursday, a mobilization that state-controlled media says is designed to guard against an American invasion. Americans focused on a U.S. military assault more likely are thinking about how President Barack Obama will pursue war in Afghanistan — not Cuba. But the siege mentality of the Cold War hasn't faded on the island, where the communist government continues to warn about imperialist aggression and the menace from the north.