Complex sentence: John's sister, Jane Slidell, was married to Matthew C. Perry, who was the Commodore of the U.S. Navy who compelled the opening of Japan to the West with the Convention of Kanagawa in 1854.
Split into simpler sentences: John's sister, Jane Slidell, was married to Matthew C. Perry. Commodore Perry of the U.S. Navy compelled the opening of Japan to the West with the Convention of Kanagawa in 1854.