In a large pot, cook the spaghetti according to package directions. Drain pasta, reserving 3/4 cup of the pasta cooking water. Set aside.
Meanwhile, in a very large skillet, cook bacon over medium heat until crisp. Transfer bacon to paper towels. Drain all but 1 Tbs. of the bacon drippings in the skillet. Add the onion and cook until tender, 6 to 8 minutes. Stir in the chickpeas and heat through. Add the spinach and cook, stirring constantly, until it wilts. Season with crushed red pepper and salt.
Add the spaghetti and toss to combine. Add enough of the pasta water, 1 tablespoon at a time, to create a sauce of desired consistency, gently stirring between each addition. Stir in the reserved bacon.