Complex sentence: The allies had been shipping supplies to Russia since the beginning of the war, through the ports of Archangel, Murmansk and Vladivostok and following the entry of the United States into the war in 1917, the Americans also began providing support.
Split into simpler sentences: The allies had been shipping supplies to Russia since the beginning of the war, through the ports of Archangel, Murmansk and Vladivostok. Following the entry of the United States into the war in 1917, the Americans also began providing support.