What happened
Presidential Decision Directive 25, finalised at the height of the killing in May 1994, set deliberately restrictive criteria for US support of UN peacekeeping operations — a policy shaped chiefly by the trauma of the October 1993 Black Hawk Down incident in Somalia, where 18 US soldiers had been killed during a UN-authorised mission.
Rather than support reinforcement of the UN mission already in Rwanda (UNAMIR), the US led the push to shrink it from over 2,500 troops to a token force of 270, arguing the mission's mandate and risk to peacekeepers were unacceptable. State Department officials were separately instructed to avoid calling the killings a genocide in public statements — declassified documents later confirmed this was because acknowledging genocide would have created pressure to invoke the Genocide Convention and act.
By the time international forces did intervene meaningfully, the killing was over. An estimated 800,000 Tutsi and moderate Hutu had been murdered in roughly 100 days — one of the fastest-moving mass killings of the twentieth century, executed largely with machetes, in a conflict where a modest, timely reinforcement of the existing UN mission is widely judged by historians to have had a real chance of limiting the death toll substantially.