Allied alliance dating
A formal military alliance is not required for being perceived as an ally—co-belligerence, fighting alongside someone, is enough.
Plus, it's not often you see a game where you can actually date a girl from an anime....
In contrast Germany had a long-standing alliance with the fading Austro-Hungarian empire dating back to the 1870s.
Other treaties (for example both sides allied to Italy at various stages) combined to ensure a tangled alliance system in 1914; by the close of the war the Central Powers had been extended to incorporate Bulgaria and Turkey.
Thus while the Entente Alliance was by no means a formal alliance, inter-twining treaties effectively rendered it thus.
The term itself was much used in 19, but was replaced by the more general 'Allies' thereafter, and was taken to include other nations including Italy and Japan.