Diderich started political life in the Liberal League, of which he was on the progressive wing. After the German occupation during the First World War, the Liberal League found itself in opposition for the first time and suffered ideological divisions. Diderich, being Mayor of Luxembourg City, took the role of leading the left-wing of the party, opposed to the leadership of the classical liberal Robert Brasseur, who had founded the party in 1904.
Diderich began his political career within the Liberal League, which was part of the progressive movement. Following Germany's invasion during World War I, the Liberal League faced a significant challenge and experienced internal conflicts. Diderich, as mayor of Luxembourg City, took on a leading role on the party's left-wing faction, opposing Robert Brasseur, who had established the party in 1904.