RECAK, Kosovo — Thousands gathered Tuesday in the southern Kosovo village of Recak to mark 20 years since 45 ethnic Albanians were slain by Serb forces in a mass killing that sparked international anger. The killings were a major factor in NATO’s decision to launch an air campaign to end Kosovo’s 1998-99 war. The Serbian government of President Slobodan Milosevic claimed at the time that the dead were all members of the rebel Kosovo Liberation Army killed in combat with state security forces.