Complex sentence: He had succeeded in bringing to power his friend and ally, but the minimal risk and cost to Argentina he had envisioned at the outset was proven over optimistic, when the resulting attack by Paraguay on Brazilian and Argentine provinces sparked the long and devastating Paraguayan War.
Split into simpler sentences: He had succeeded in bringing to power his friend and ally, but the minimal risk and cost to Argentina he had envisioned at the outset proved to be illusory. The resulting attack by Paraguay on Brazilian and Argentine provinces sparked the long and devastating Paraguayan War.