Israeli Prime Minister Benjamin Netanyahu is warning those who want to destroy Israel and the Jewish people that they will be destroyed themselves.
Netanyahu spoke at Jerusalem's Yad Vashem memorial to 6 million Jews on Holocaust Remembrance Day.
"Iran and the Islamic State want to destroy us, and hatred for Jews is being directed towards the Jewish state today," Netanyahu said. "We must be able to defend ourselves by ourselves against all threats and any enemy."
He said this lesson from the Holocaust guides him every day and that lesson is also the "supreme duty" of every Israeli prime minister.
But he said there are many cases in which the world did not learn the lessons from Nazi Germany's attempt to wipe out the Jewish people.
Sunday's ceremony included six Holocaust survivors lighting six torches while the traditional Jewish prayer for the dead, Kaddish, was recited.
In New York, President Donald Trump told the World Jewish Congress that the mind "cannot fathom the pain, the horror and the loss" of the Holocaust.
"We look back at the darkest chapter of human history. We mourn, we remember, we pray and we pledge -- never again. I say it, never again."
U.N. Secretary-General Antonio Guterres told the congress that the modern form of anti-Semitism is the denial of Israel's existence.
He said Israel needs to be treated like any other U.N. member state.
The Security Council voted in December to condemn Israeli settlement activity as harmful to the peace process. The U.S, under former president Barack Obama, abstained rather than vetoing a resolution critical of its most important ally in the Middle East, angering Israel.
Israel's annual Holocaust remembrance continues Monday morning when at 10 o'clock local time, sirens will wail as the entire country stands in two minutes of silence for the victims of Hitler's genocide.