The Nuremberg Trials at the end of World War II were an important event for international human rights during warfare. Many of the leading Nazis were put on trial and most were found guilty of carrying out horrific crimes—the most heinous being the murder of six million Jews. But the victors of the war had also committed war crimes. The Soviet Union's crimes included the slaughter of thousands of captured Polish officers during the war. The US killed thousands of civilians with aerial bombing and with nuclear weapons. Nevertheless, the Nazis were truly evil, and the Nuremberg Trials could have been the start of something more permanent.
The US arguably violated international law and human rights norms with its wars in Vietnam and Iraq. In addition, the CIA was involved in the torture of terrorism suspects after 9/11, but no Americans were ever brought to trial at an international court. Today, the United States does not submit to the authority of the International Criminal Court (ICC) in the Hague, Netherlands. Because the government insists that Americans are not subject to a judicial body that enforces international human rights laws, America cannot truly be considered the chief defender of human rights in the world today.
The US has fought for human rights intermittently. The US had cordial relations with many dictatorial regimes during the Cold War. Under President Jimmy Carter, human rights were an extremely important part of American foreign policy. Many argue that under Donald Trump's leadership, the US has moved further away from a global leadership role with regard to human rights. For instance, Trump has forged a close and personal relationship with the leaders of repressive regimes in North Korea and Saudi Arabia.