I've seen this "strategy" as far back as Civ 2 or Civ 3: an AI builds up troops but doesn't attack anyone; then as the troops start to become obsolete due to general advance in tech, they declare war just to burn up their unit maintenance costs and maybe get some XP points for the survivors.
If you withstand this cannon fodder type of attack, they are typically very weak when you counter-attack: at the slightest threat to their capital, they will gift you a city as a peace proposal. In some cases, you can request a different city and get it. Although the "gift" brings undiminished city population, some useful buildings not destroyed by war, maybe some Great Works or even Wonders, it can also be a lot to digest in terms of your treasury budget, happiness level, and progress towards Great People.
I even wondered some years ago if this "easy giveaway" was intended by the game designers as a kind of blowback/revenge to undermine the happiness of the war's winner.
If you withstand this cannon fodder type of attack, they are typically very weak when you counter-attack: at the slightest threat to their capital, they will gift you a city as a peace proposal. In some cases, you can request a different city and get it. Although the "gift" brings undiminished city population, some useful buildings not destroyed by war, maybe some Great Works or even Wonders, it can also be a lot to digest in terms of your treasury budget, happiness level, and progress towards Great People.
I even wondered some years ago if this "easy giveaway" was intended by the game designers as a kind of blowback/revenge to undermine the happiness of the war's winner.