Complex sentence: The Type 201 was Germany's first class of military submarines built after World War II. They were built out of amagnetic steel to counter the threat of magnetic naval mines, but the material had been insufficiently tested and proved to be problematic in service with the Bundesmarine.
Split into simpler sentences: The Type 201 was Germany's first class of military submarines built after World War II. They were built out of amagnetic steel to counter the threat of magnetic naval mines. But the material had been insufficiently tested and proved to be problematic in service with the Bundesmarine.