WASHINGTON — On April 12, 1994, six days after a plane carrying Rwanda’s president was shot down, setting off a wave of killings, Madeleine K. Albright, the American ambassador to the United Nations at the time, sent a cable to the State Department proposing that the United States take the lead in pushing to withdraw most of the United Nations peacekeeping force operating there. Ms. Albright’s tersely worded cable, recently declassified, starkly captures the reluctance of the United States to respond to the deepening crisis in Rwanda. When most of the United Nations force was withdrawn shortly afterward, leaving the...