Complex sentence: In 1906, he was promoted to major but retired the following year and transferred to the foreign service but had to leave in 1908 due to his engagement with the Archduchess.
Split into simpler sentences: In 1906, he was promoted to the post of major, but retired the following year and transferred to the foreign service. However, he had to leave the service in 1908 due to his engagement with the Archduchess.