In Afghanistan against the U.S., perhaps, with insurgents often taking refuge across the Pakistani border.

But the Iraqi insurgency didn’t have much outside state support, and was based entirely in Iraq.

The Pakistani Taliban against Pakistan doesn’t have outside state support, rarely (if ever) leaves Pakistan, and has persisted against the Pakistani military for many years.

Russia, despite a willingness to use brutal force, hasn’t fully suppressed militant groups based in the caucuses. I’m not aware of them getting outside support.

There are other examples.

It’s better for insurgents if they have outside support and sanctuary, of course. But at least in the 21st century, it doesn’t appear necessary for them to last a while. Note that I didn’t say win. Just “remain active for a long time.”