In a large skillet or saucepan, toast the chiles on each side for about 10 seconds, or just until they begin to puff. Cover the chiles with enough water to submerge them, bring the water to a boil, and then turn off the heat. Allow the chiles to soften in the water for approximately 30 minutes. Once the chiles have soaked, remove them from the water, remove the stems, and place them in a blender.
Heat 1 tsp. of oil in a large pot/dutch oven over medium heat. Add the onions and cook until translucent, for approximately 5 minutes. Add the garlic and cook for 30 more seconds.
Add the cooked onions and garlic to the blender with the chiles. Add the tomatoes, 1 cup of broth, chili powder, cumin, allspice, and cayenne. Puree the mixture until smooth.
In the same pot you used to saute the onions and garlic, add the other tsp. of oil and heat over medium heat. Once oil is hot, add the fideo and cook for 2 minutes until it becomes a little toasted and brown, ensuring that you stir constantly.
Add the chile/tomato mixture to the pot with the pasta. Add approximately 5 more cups of broth. Bring the sopa to a boil over high heat, reduce the heat to medium and allow the pasta to simmer for approximately 10 minutes until it is tooth-tender. If the broth thickens or disappears too much for your liking, feel free to add more chicken broth.
Taste for seasoning and add salt and pepper as needed.
Serve fideos in a bowl with cilantro and a lime wedge. You can add a sprinkling of Mexican cheese (such as queso quesadilla or queso fresco) when serving as well.