Albus knows he is the only one who can win this fight. Gellert's rein has gone on too long. One-shot Dumbledore vs Grindelwald
The war is over, however not all went to plan. Many died. Too many. Harry and his friends are some of the few survivors of the Order of the Phoenix, and after the war, attempt to live normal lifes. However, it seems fate doesn't want them to. Post-OOTP
Pre-HBP. Sirius is gone. Harry disappears. The war is starting to get worse. They need their heroes, which is exactly what they will get.