Complex sentence: During the Siege of Paris, the only way for the provincial government in Tours to communicate with Paris was by pigeon post, and, as the pigeons could not carry paper dispatches, the Tours government turned to microfilm.
Split into simpler sentences: During the Siege of Paris, the only way for the provincial government in Tours to communicate with Paris was by pigeon post. As the pigeons could not carry paper dispatches, the Tours government turned to microfilm.