Complex sentence: On receipt of MacDonald's message, Seymour assembled within 24 hours a lightly armed force of 2,000 sailors and marines from Western and Japanese warships and prepared to embark for Beijing from Tianjin, 75 miles away, by train.
Split into simpler sentences: On receipt of MacDonald's message, within 24 hours, Seymour assembled a lightly armed force of 2,000 sailors and marines from Western and Japanese warships. The sailors and marines embarked for Beijing from Tianjin which is 75 miles away, by train.