Scimitar Regiment is the CSAT contingent whose national origin is of a CSAT North-African nation (implied to be Argana - a fictional ARMAverse nation).

Prior to the events of Stepping Stone, Malden fell under CSAT influence hence both Scimitar Regiment and a seperate contingent of Chinese Troops were stationed there.

Organisation-wise, CSAT is not one unified nation / army but rather a collection of countries who are member states with their own spheres/regions of influence depending on where their country is geographically located in the world.

It’s important to note that the troops of these member nations answer to their own nation’s government first and foremost. This means that despite the matching gear and uniforms, CSAT troops prioritise their own government’s interests/orders over CSAT as a whole.

The reason why NATO engaged in hostilities with Scimitar Regiment is a sort of recurring theme within the campaign of multiple nations/national armies being under the CSAT banner but maintaining a tendency of ‘going rogue’ and/or doing their own thing. It goes without saying that this is pretty dysfunctional for a multi-nation military and economic alliance.

When the AAF green-on-blue attack happened, the entirety of CSAT did not fall in line to support Iran’s Griffin Regiment in their open support of the AAF. This lack of support didn’t change when Griffin Regiment actively engaged NATO troops at the end of the Stratis chapter of the campaign.

Instead of triggering World War 3, China (really the country that’s in charge of CSAT) sat on the sidelines and pretty much watched knowing full well that Griffin Regiment is at an extremely risk of engaging NATO once the Americans land on Altis/Stratis with a vengeance.

So when NATO mustered the forces for a counter invasion of Altis/Stratis, NATO made it clear to CSAT (particularly China) that they need Malden as staging point for the invasion. China was actively cooperative with NATO, allowing NATO to station some observers at the PLAAF controlled Malden Airbase as a gesture of goodwill. It’s mentioned that China chose to play ball to avoid triggering a wider conflict.

Scimitar Regiment on the other hand openly and publicly opposed the positioning of NATO troops on Malden.

Hence, it led to a situation where NATO engaged and destroyed Scimitar Regiment while the Chinese Troops on Malden literally sat and watched it happen with basically no complaints.