Walter Mittelholzer was killed in a climbing accident in 1937. Balz Zimmerman died unexpectedly only a few months later. Swissair, however, survived the deaths of its founders and continued to operate successfully throughout Europe. Yet Swissair was forced to cancel all operations indefinitely at the end of August, 1939. Nazi Germany had started a war in Europe which neccessitated the closure of all air services. Switzerland, which was determined to maintain strict neutrality, waited nearly six years for the war to end.