Complex sentence: At first, Greece supported the French-British military support of Serbia, then they opposed it; finally, after Venizelos' resignation, the royalist government settled for officially condemning it, but not actually opposing the superior Allied armies that had landed in Salonika.
Split into simpler sentences: At first, Greece supported the French-British military support of Serbia, but after the Allies landed in Salonika gradually changed policy. With the Venizelos' resignation, the royalist government settled for officially condemning it, but was unable to oppose the superior Allied armies that had landed in Salonika.