Complex sentence: Within two years of Spring-Rice's posting to Washington, D.C., the First World War had broken out in Europe, and his principal concern became working towards ending American neutrality, which was achieved with the US's entry into the conflict in 1917.
Split into simpler sentences: Within two years of Spring-Rice's posting to Washington, the First World War had broken out in Europe and his principal concern became working towards ending American neutrality. This was achieved with the US's entry into the conflict in 1917.