Every mission inflicts (a will debuff). If you send a Tired soldier on a mission, they risk becoming Shaken (huge aim/will penalty) or even Deserting .
In vanilla, gravely wounded means 20 days out. In Long War, a soldier who uses two abilities in a mission might be "Tired" for 8 days. A soldier who uses no abilities is ready for the next mission in 2. Meta-strategy: Build a roster of 40 soldiers. Your "A-team" is a myth. You have an "A-pool" of 12, a "B-pool" of 15, and a "C-pool" of sacrificial Shinobis who only run overwatch. xcom 2 long war guide
Crucial for training your initial rookies into specific classes to avoid "bad" RNG rolls. Understanding the Long War Mod
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The "Golden Path" (story missions) remains, but the focus shifts to liberating regions Find a "Lead" Role: Removed the pistol tree