The engines, big and small, came from all over the country to fight the Kincade fire in the Sonoma county wine region of California. There were trucks from Nevada, South Dakota, Colorado — and from the wildfire protection unit of home insurer AIG.

As fires have increasingly encroached on development in California’s wildlands in recent years, communities are grappling with a new paradigm of risk.

If the fire creates an existential crisis for people living in high-risk areas, it also creates one for the companies that insure their homes.

Insurers of houses, timber and agriculture have contracted with private firefighting agencies for decades. But now, thanks to longer, more devastating fire seasons, the business is booming.

While some wealthy communities and individuals have contracted their own private firefighting services to defend mansions on hilltops from flying embers, the majority of these agencies work on behalf of insurance companies.