[2]: 5  After starving himself, Levi was discharged on medical grounds in 1916, settling in Switzerland and associating with Karl Radek, Grigory Zinoviev and Vladimir Lenin, becoming a part of the bureau of the Zimmerwald Left and helping found La nouvelle internationale which he wrote for under the pseudonym 'Hartstein'.

These efforts were rewarded when a substantial section of the USPD joined the KPD after a debate at their Halle congress, making it a mass party for the first time with around 449,700 members.

[2]: 13  This was followed up by an 'Open Letter' which Levi, alongside Radek, convinced the KPD Zentrale to issue to other working class organisation to join in a joint struggle around their common interests, based on a successful initiative of Communists in Stuttgart.

[2]: 15 Levi attended the 1921 Livorno Congress of the Italian Socialist Party (PSI) which had joined the Comintern, where Levi had supported Giacinto Serrati against the faction around Antonio Gramsci and Amadeo Bordiga who went on to form the Italian Communist Party (PCI) supported by Comintern representatives Mátyás Rákosi and Khristo Kabakchiev.

[2]: 18 As leader of the KPD, Levi frequently criticized "putschism," or the repeated efforts on behalf of Communists to take power without the broader support of the masses.

Levi cited both Engels and Marx on insurrection, showing how other Party members of the KPD had overlooked the careful preparatory work of the Bolsheviks leading up to the October 1917 Revolution.

He simultaneously criticized Comintern representatives such as Béla Kun, Grigory Zinoviev and Karl Radek for their encouragement of these accelerationist policies both in Germany and Italy.

[2]: 24 Levi began rethinking his previous policies and wrote introductions to Rosa Luxemburg's The Russian Revolution and Leon Trotsky's Lessons of October, which were sharply critical of the Bolsheviks.

[2]: 25–26  In the summer of 1921 Levi founded a monthly magazine named Unser Weg ("Our Way"), which he later replaced with a weekly Sozialistiche und politische Wochentliche (also known as Levi-Korrespondenz) when he rejoined the SPD.

He responded by attacking prominent Nazis, such as Adolf Hitler, Ernst Röhm, Alfred Rosenberg, and Wilhelm Frick in left-wing publications.

[2]: 28  Levi also began to specialise in defending writers and newspapers that disclosed government secrets, but took up other civil liberties cases, including those of KPD members such as Willi Münzenberg.