system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - If we're supposed to be evacuating our (US) troops from Afghanistan, why are we still shipping new troops over?

Context - I have two friends who are relatively high-ranking officers in the Air Force that have just been shipped over and they're either unwilling or unable to tell me the answer to this question.
gpt
It has to do with the fact that military personnel are only deployed for limited amounts of time (tours) that are shorter than the overall war/withdrawal schedule.

Lets pretend there's 9,000 troops over there right now and they're supposed to be completely gone in 3 years at even intervals.  So after 1 year we only have 6,000 there, in 2 years it's down to 3,000 and at the end it's down to 0.

However you don't want to have 3,000 soldiers stuck there for 3 straight years.  So what you might do is after year 1 you pull back 6,000 troops and send back in 3,000 meaning you're still at 6,000 total but 3,000 of them are new.  Then at the end of year 2 you pull out the 3,000 that have been there the longest.  That way you're not sticking one group with staying there for 4-5 years straight for the pullout.

TL;DR: Troops get sent in to replace troops coming home but they're sent in at a slower rate so the overall number actually decreases.


Note: If they're actually high-ranking officers, then the rules are completely different as the presence of higher ranking personnel is usually based on various command needs rather than a real timeline.