Complex sentence: Having been involved in the organization of the strike, Prieto fled to France before he could be arrested; he did not return until April 1918, by which time he had been elected to the Spanish Congress of Deputies.
Split into simpler sentences: Having been involved in organizing the strike, Prieto fled to France before he could be arrested. He did not return until April 1918, by which time he had been elected to the Spanish Congress of Deputies.