Quick Play Low Intensity is a set of free wargames rules for playing campaigns set in Vietnam or similar conflicts.
Masters of the Mediterranean is a set of campaign rules for ancients.
The always amazing Perfect Captain has released A Coat of Steel, a set of free wargames rules for the Wars of the Roses. There’s also a campaign game called “A Crown of Paper.”
Beyond the Saale is a set of campaign rules for the Seven Years War.
Running a “campaign” can add context and interest to a miniature wargame. Here’s a Tobruk Desert Campaign for Flames of War.