Complex sentence: German colonial postal services ceased at varying times during WWI as colonies were occupied by forces of the Allies of World War I. During allied occupations during WWI, overprinted German yacht series stamps were used for a while in a number of territories.
Split into simpler sentences: German colonial postal services ceased at varying times during WWI as colonies were occupied by forces of the Allies. During occupation, German yacht series stamps were used for a limited time with overprints in a number of territories.