Complex sentence: A state of war had existed since 9 March 1916 between Germany and Portugal: when neutral Portugal complied with a British demand to confiscate German ships interned in Portuguese ports, Germany reacted by declaring war on Portugal.
Split into simpler sentences: A state of war existed since 9 March 1916 between Germany and Portugal. After neutral Portugal complied with a British demand to confiscate German ships interned in Portuguese ports, Germany reacted by declaring war on Portugal.