Complex sentence: The Reich Chancellery bunker was initially constructed as a temporary air-raid shelter for Hitler (who actually spent very little time in the capital during most of the war), but increased bombing of Berlin led to expansion of the complex as an improvised permanent shelter.
Split into simpler sentences: The Reich Chancellery bunker was initially constructed as a temporary air-raid shelter for Hitler (who actually spent very little time in the capital during most of the war). Increased bombing of Berlin led to expansion of the complex as an improvised permanent shelter.