Answer:
To arrest those disloyal to mexico
Step-by-step explanation:
General Cos was the brother-in-law of Santa Ana. Both got along well and shared very similar political opinions, which made Cos share Santa Ana's idea of canceling the Texan constitution and ending the state's autonomy.
The cancellation of the Texas constitution triggered the Texan revolution, which made both Santa Ana and General Cos consider the Texans as traitors and disloyal to Mexico. For this reason, Cos decided to take a large military force to Texas, to arrest all the disloyal to the country and that was what he did, defeating several Texan generals.