My 3-year-old german shepherd barks at everything he sees through the window — people, cars, dogs, everything. I've tried closing the curtains but he just barks

Block visual access to the window entirely. Use blackout curtains, move furniture, put up cardboard — whatever it takes. He's been rehearsing this behavior for years and the environment is feeding it. You can't train him out of it while he's still practicing it 10 times a day. Once the window is blocked and he can't see triggers, work on the underlying alert behavior. When he hears a noise and alerts (perks ears, looks toward the door), immediately redirect him to a mat or bed and reward him for lying down. You're building a new routine: hear trigger → go to mat → get reward. After two weeks of no window access and consistent mat work, reintroduce visual access in short controlled sessions. Let him look out the window for 30 seconds. If he's calm, reward him. If he barks, block the window again. Gradually increase duration. Most GSDs need 4-6 weeks before they can handle an open window without losing their mind. If you skip the management phase and try to train through the behavior, you'll fail.