Arriving in Munich, he contributed to Dietrich Eckart's publication, the Völkischer Beobachter (Ethnic/Nationalist Observer). By this time, he was both an antisemite – influenced by Houston Stewart Chamberlain's book The Foundations of the Nineteenth Century, one of the key proto-Nazi books of racial theory – and an anti-bolshevik.[11] Rosenberg became one of the earliest members of the German Workers' Party – later renamed the National Socialist German Workers' Party, better known as the Nazi Party – joining in January 1919, eight months before Adolf Hitler joined in September. According to some historians, Rosenberg had also been a member of the Thule Society, along with Eckart,[12] although Nicholas Goodrick-Clarke contends that they were only guests.[13][14] After the Völkischer Beobachter became the Nazi party newspaper in December 1920, Rosenberg became its editor, in 1923.[15] Rosenberg was a leading member of Aufbau Vereinigung, Reconstruction Organisation, a conspiratorial organisation of White Russian émigrés which had a critical influence on early Nazi policy.[16]