The squirrel crosses the path and your dog is gone in a blink. Chasing can be strongly rewarding on its own, and no cue becomes reliable by being shouted after a dog is already sprinting. Safety has to come before the training exercise.
Keep your dog on a secure lead in places with wildlife, roads, or other hazards. A long line can give room to sniff in an appropriate open space, but not beside traffic or where it can tangle around people, animals, or trees. Check the fit of the harness and choose walks where you can see the path ahead. Preventing a chase today is not admitting defeat, it stops your dog practicing the very behavior you want to change.
Train before the squirrel appears
At home, reward your dog for turning to their name or a clear recall cue. Make coming to you worth it, sometimes releasing them again to sniff rather than ending every good thing. Then practice in a quiet outdoor place on a secure line, away from wildlife. Start with easy distances and distractions before moving on.
When you notice a squirrel at a distance, call your dog once while they can still respond. Reward a turn toward you and move away together. If they stare hard, pull, or ignore food, add distance rather than repeating the cue. You may need to take another route. Give your dog other safe outlets for seeking and chasing, such as sniffing games or a controlled toy game, while keeping wildlife out of it.
If your dog has pulled free, injured themselves, or is strong enough to pull you over, ask a qualified, reward-based trainer for hands-on help. A safe, enjoyable walk does not require your dog to sit still inches from a squirrel. It requires a plan that works before the sprint begins. For more on managing chasing safely, see the dog chasing guide, the secure lead remains the immediate safeguard around wildlife and road
Sources: PDSA: How to tame prey drive in dogs. Dogs Trust: How to stop your dog chasing cars, bikes and animals.