Haggar is a Side eastward of Jetstone. Its relationship with Jetstone has traditionally been troublesome. They originally hoped to exploit Jetstone's troubles by annexing one or more of its cities. However, Chief Warlord Ossomer succeeded in intimidating Haggar to the point where they agreed to join the Royal Crown Coalition against Gobwin Knob. Haggar's leaders secretly planned to betray this alliance at a time most beneficial to themselves. However, Prince Sammy has begun to wonder if perhaps Jetstone is indeed right about the threat posed by Gobwin Knob. Either way, his hand was forced by Charlie, who has set up an ambush around the City of Haggar and threatened to sack it unless Haggar didn't turn on Jetstone and attacked Gobwin Knob's forces.
Real World References
Haggar seems to refer to the clothing company by the same name.
Prince Sammy Haggar is based on the performer Sammy Hagar.
The Haggar emblem is the logo for Cabo Wabo tequila, which is owned by Sammy Hagar. "Wabo" is one of the war cries of Sammy's squad.
Haggar may also refer to the comic strip (and titular character) Hagar the Horrible.
Haggar was the name of an enemy witch in the cartoon Voltron, though there are no obvious references to this so far.
Likewise, Haggar is the name of a Capcom character who was a professional wrestler and mayor (and which, merely incidentally, favored pants as his only clothing), but there is no clear reference.