I find Pub Battles uniquely suited for playing alone. Note, it is not a “Solitaire game” per se. It does lend itself quite readily to playing sans opponent, however.
The rules are fairly simple and intuitive, so you’re not trying to keep all kinds of details in your head, nor referring to endless charts.
The chit draw mechanic lends itself to a “quick save” option if you have to leave quickly to handle some Real Life event.
The game plays so smoothly that it is possible to play out many what if scenarios. Command Post games even gives you all the participants in the Waterloo campaign so you can try out different possibilities for who might have made it when to Waterloo. They provide you with an alternative “What if Jackson had survived” scenario that lets you fight Gettysburg as if Lee had his “Stonewall” beside him! Of course, you can always start with Stuart appearing right away.
Finally, whenever I play alone, I orient the map so one side is facing me, and then I turn the other side’s pieces around so that I can easily see who they are even while fresh.