What causes changes in alien troops?

[llll BlackFlag]

As the title states, I'm curious about what mechanic causes advent troops to level up to M2/M3, or to start recruiting more aliens. Is it vigilance, regional strength, calendar date, a combination? In my current campaign I'm fighting heavy mecs and mutons in a strength 2 region, and garden variety of advent + sectoids and sneks in a strength 3 region (both during extremely light jailbreak missions). Had I known the troops would be stronger in the strength 2 region I would have changed my squad around

[Psieye]

Effectively it's calendar date. Technically it's Force level and you can theoretically catch a Force-increasing UFO and deny that, but assume you'll never see one ever.

[llll BlackFlag]

Effectively it's calendar date. Technically it's Force level and you can theoretically catch a Force-increasing UFO and deny that, but assume you'll never see one ever.

Thanks for the quick reply. Ufopaedia says the force level should always be the same in all regions. If that's true, why am I getting drastically different pod compositions on similar missions? Also, do you know if there is a way to tell the difference between force increasing and alert increasing UFO's based on the mission screen?

[fowlJ]

Effectively it's calendar date. Technically it's Force level and you can theoretically catch a Force-increasing UFO and deny that, but assume you'll never see one ever.

Thanks for the quick reply. Ufopaedia says the force level should always be the same in all regions. If that's true, why am I getting drastically different pod compositions on similar missions? Also, do you know if there is a way to tell the difference between force increasing and alert increasing UFO's based on the mission screen?

Not all old aliens stop appearing when new ones are introduced. You can still keep seeing M1 ADVENT Troopers for a while, for instance, and M2 Troopers stick around until the end of the campaign. Sometimes you just don't happen to roll any of the scary ones on a mission.