Short answer is they don’t fight. And they also make themselves targets for terror acts. Johnny Taliban knows who has strong support at home and which countries will pull out if they can just killing a couple dozen of it’s soldiers with an IED.
McChrystal Plan #1: Win.
McChrystal Insight #1: Don’t count on the allies to (a) guard their sector, (b) fight their sector, (c) win hearts and minds in their sector, (d) pay their share, (e) equip their share, (f) or do anything at all.