ISIS conduct multiple beheading in one go

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How is ISIS slowing down or being defeated? They've just about opened a new front much closer to Europe in Libya, They've opened a new front in Yemen, and there are traces of ISIS in Afghanistan/Pakistan now.
The only way to stop/defeat ISIS is a proper ground invasion and a coalition from America/Russia to make it illegal to sell weapons to any shady charecters(100% of ISIS weapons are American and Russian origin)

How would they manage this? And no they're not 100% American/Russian origin, I think you mean DESIGN. For example there are small arms manufacturing plants in Egypt building from American and Russian blueprints.

Most of those weapons have a long and complicated trail. It isn't like ISIS just had a guy skype the pentagon in a fake mustache and order guns. Most of those were at some point in the hands of a legitimate military in the region, which then saw several splinter groups and sectarian disputes etc... ISIS probably has a lot more guns than it has personnel to use them given the chunk of Iraq they took.

The weapons are not only of mixed origin but also mixed generations, stuff 50 years apart in manufacture, Mosin-Nagants alongside FN FALs alongside Chinese SKS.

It's nearly impossible to control the flow of small arms even in a developed country (it's easy to get a gun even in countries where they're "banned" -- barely more difficult than buying weed), much less countries with no central government.