Complex sentence: In one of his letters he wrote of the fantastic expansion of trade and imperialism in India and America, but also that the Republic then had for 150 years had more soldiers than all the other Christian countries put together.
Split into simpler sentences: In one of his letters he wrote of the fantastic expansion of trade and imperialism in India and America. He also wrote that the Republic then had for 150 years had more soldiers than all the other Christian countries put together.