Roofuskit hid the main answer to your question (within a great commentary on US wildfire): Fires require 3 things, oxygen, fuel, and heat.

Remove the fuel source in front of a fire, and you prevent its spread.

In the western US, this is done to create a fire break - a line which a wildfire can't cross because there's no fuel.

And the rest of what roofus said is very important: US policy around fires for the last 80 years or so has been to suppress ANY fire. This has had the opposite effect of enabling more undergrowth to form, meaning more fuel.

Additionally, California in particular essentially stopped all logging. Logging companies maintained what we commonly call fire roads - those roads were often an unintentional fire break. Those roads overgrew with the kind of growth (low scrub) that is ideal wildfire growth fuel.

Wildire in the western US is a lesson in assuming we understand how natural systems work, and have the ability to control them.

Edit: firefighters in a city/town remove oxygen when using fire suppressant materials. Water is used to remove heat when the fire turns it to steam.

CO2 is largely what's in most consumer fire bottles - it provides an oxygen-free "cloud" to break the cycle of a fire by replacing the available oxygen.