Complex sentence: The main flight base was established in Horten and other bases were established in Kristiansand in 1918, Bergen in 1919 and in Tromsø shortly before the invasion in 1940.
Split into simpler sentences: The main flight base was established in Horten. Other bases were established in Kristiansand in 1918, Bergen in 1919 and in Tromsø shortly before the invasion in 1940.