FatLanders met some resistance to their beliefs that dieting was unhealthy and caused people to gain weight instead of losing it, but they didn't meet much resistance to their settling in what were previously unsettled park and desert and mountain valleys.
What they did meet was resistance from some of the Big Diet Corporations and from the owner of a multi million dollar concern who, ironically, was bankrolling a lot of their efforts. He sent drones flying over FatLand twice. He wanted to intimidate FatLanders into giving into his demands (different demands in each instance). Of course he failed.
FIN -the FatLand Intelligence Network- was created in response to his efforts. They also created and built the Safe House complex for FatLanders perceived to be in danger from his efforts to undermine and sabotage FatLand's policies (perhaps even more than its buildings and roads and infrastructure, which he also had a hand in building, paradoxically). FIN also played a key role in designing and often carrying out the rescue of people on the Other Side who were in danger of being sent to Re-Education Centers during the period of the Health and Diet Laws.
The war fought against FatLand was mostly one of stealth - perhaps the most dangerous kind of war of all.