Complex sentence: Adolf Hitler decided to visit Finland on 4 June 1942, ostensibly to congratulate Mannerheim on his 75th birthday, but Mannerheim did not want to meet him at his headquarters in Mikkeli or Helsinki, as it would have seemed like an official state visit.
Split into simpler sentences: Adolf Hitler decided to visit Finland on 4 June 1942, ostensibly to congratulate Mannerheim on his 75th birthday. But Mannerheim did not want to meet him at his headquarters in Mikkeli or Helsinki, as it would have seemed like an official state visit.