Place the chicken breasts in the bottom of your slow cooker.
Scatter the sliced onion and bell peppers over the chicken.
In a small bowl, mix the fajita seasoning, cumin, chili powder, and garlic powder.
Pour the diced tomatoes and green chilies (undrained) and chicken broth over the contents of the slow cooker. Sprinkle the seasoning mix evenly over everything.
Drizzle the olive oil on top.
Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the chicken is easily shredded.
Remove the chicken from the slow cooker and shred it using two forks. Return the shredded chicken to the slow cooker and stir to coat it with the juices and vegetables.
Serve hot with warm tortillas and your favorite toppings.
Notes
For best results, use low-sodium seasoning and broth to control the salt level.
If you prefer less liquid, drain off some of the broth before shredding the chicken.