If you want to play the yanks against AI rebs you will probably have to modify the scenario header. The longer scenarios normally do not have scripts, they are both tedious to create and do not work well for long scenarios, especially multi-day ones. The majority of the longer scenarios do not have the AI set to attack for either side, the exceptions being the historical scenarios. If you open a scenario with the editor and look at Scenario/Header you will see a box labeled AI Values. These set the values that determine how the AI acts.

The Help file explains the AI Values

The A/I Values are used when a side in a scenario is played using Automatic A/I.
An A/I value of 0 indicates that the corresponding side should conduct an Extreme Defense in the scenario.
An A/I value of 1 indicates that the side should conduct an ordinary Defense.
An A/I value of 2 indicates that the side should not initiate any A/I orders on its own.
This value is best used when A/I Scripts are going to be defined for the scenario.
An A/I value of 3 indicates that the side should conduct an Attack.
An A/I value of 4 indicates that the side should conduct an Extreme Attack.
If an A/I Value different from 2 is defined for one or both sides, then the Flags value will be important.
A flag value of 1 indicates that the A/I should not commit the entire side to the Attack or Defense, but should only commit forces based on opportunities.
A flag value of 0 will cause the A/I to commit the entire side to the Attack or Defense.

If you see both sides set to 0 the AI will sit and only react to your moves. This means if you want to play the rebs you would probably be ok but if you want to play the yanks the rebs will not initiate much in the way of attacks.
If you wish to modify these values look at the Help file, Page 5, it explains how to go about modifying a scenario without messing up the stock scenarios.

Have you had sight of the 2023 Tournament Challenge recently posted here. 'Stonewall at Gettysburg' may very well be worth a look. Definitely not historical but unfortunately for you it's a tourney, so 1 on 1 as opposed to playing the AI.