As with Otto von Bismarck's Three Emperors' Alliance and later the Dual Alliance, Franco-Russian Alliance, Anglo-French Entente, and Anglo-Russian Agreement.
European diplomacy can be summarized in a single word: 'balance'.
If one stands out, others rush in to smash their head with a hammer.
If word spreads that someone struck gold, others immediately jump into competition.
Perhaps this European diplomatic history has continued since medieval times when popes excommunicated overbearing kings.
Unlike East Asia, there seems to be more of this tendency since ethnicities and languages weren't historically clearly distinguished.
Anyway, in this era where everyone acts like they're the guardian of balance, peering here and there, increasing military expenditure is in a way logical.
European countries' military spending has increased 50-80% from five years ago to this year.