Israel launched an airstrike and crossed the border into Gaza with tanks and bulldozers after Palestinian militants detonated a bomb by an Israeli army patrol, killing one and wounding several others. The violence is the first following a week-long truce between Israel and the Hamas government of Gaza, and threatens the push for a permanent peace. The new round of fighting comes only one day before George Mitchell, the new U.S. Mideast envoy, visits Israel. He arrived in Egypt on Tuesday to meet with officials who have been mediating between Israel and Hamas.