The anniversary of the so-called "Anschluss" comes as a new conservative-led coalition government in Austria includes the far-right Freedom Party (FPO), founded by former Nazis in the 1950s.
Austria's president warned against the rise of far-right ideologies on Monday, as the country marked 80 years since its annexation by Hitler's Germany.
Speaking in the former imperial palace where Hitler addressed a crowd of hundreds of thousands to announce the so-called "Anschluss" on March 12, 1938, President Alexander Van der Bellen said the annexation had opened "the darkest chapter in the history" of Austria.
Van der Bellen has repeatedly called for vigilance against populist ideas since conservative Chancellor Sebastian Kurz took office in December. Kurz's coalition government includes the anti-immigration Freedom Party, a party founded by former Nazis in the 1950s which now focuses its criticism on Islam.
"The German Wehrmacht came overnight. But disdain of democracy, disdain of basic rights and freedoms didn't come overnight, militarism, intolerance and violence didn't come overnight. They had settled in Austria over time," Van der Bellen said.
"Austria has its share of responsibility for the atrocities of Naziism. Austrians were not only victims, but also perpetrators, often in leadership positions."
Van der Bellen said Austrians must learn "the lesson that democracies too can fall victim to populism and demagogy, the lesson that discrimination is a first step towards dehumanisation, the lesson that anti-Semitism and racism do not simply disappear but exist even today in big and small ways."