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You need to have a Friendly relation with the target side, and then you can click on the + sign in the war overview (diplomacy page). If it's greyed out, there's a tooltip that tells you what you're missing.

Either that, or ally them.
 
Yea, I allied and sphered them but didn't see an option on Texas' diplomacy page, or under Declare War on Mexico. Thanks for the tip! I'm off to march to Sinaloa!
 
Yea, I allied and sphered them but didn't see an option on Texas' diplomacy page, or under Declare War on Mexico. Thanks for the tip! I'm off to march to Sinaloa!

I think you can also only intervene on a losing side of a war, so if Texas hasn't lost any territory yet, you may not be able to help them out until they do.
 
you can only intervene if texas is losing. If its not losing, no intervention is possible.