What event is Hitler referring to when he repeats the phrase ‘thirteen years ago?’

The event that Hitler is referring to when he repeats the phrase "thirteen years ago" is the Treaty of Versailles.

Explanation:

The Treaty of Versailles was concluded between the Allied nations and Germany after the First World War on June 28, 1919, in which Germany was convicted alone of war guilt and agreed to pay substantial war reparations.  

World War I ended with the surrender of Germany in November 1918. After that, negotiations on peace terms began. The terms of the Treaty of Versailles were negotiated at the six-month Paris Peace Conference in 1919. At the same peace conference, peace agreements were also negotiated with Bulgaria and the Ottoman Empire.  

As early as January 8, 1918, President Woodrow Wilson presented a Fourteen-Point Program that sought to address the causes of the war, with no specific penalties for Germany. However, when peace was reached in early 1919, France and Britain demanded harsh peace terms for Germany, and the final Peace of Versailles in 1919 was in line with French and British demands.

Historians have since agreed that the Peace of Versailles did not solve Europe's problems. The Treaty of Versailles sowed the seeds of World War II and was the main cause of World War II, as it generated great resentment in German society towards France and Great Britain, which due to the Treaty of Versailles had imposed great economic and territorial sanctions that humiliated their national sentiment. This situation prompted the birth of the nazi movement, led by the authoritarian and racist Adolf Hitler, who reached power years later.