For a century, the Appalachians were a barrier to the westward expansion of the British colonies. The continuity of the mountain system, the bewildering multiplicity of its succeeding ridges, the tortuous courses and roughness of its transverse passes, a heavy forest, and dense undergrowth all conspired to hold the settlers on the seaward-sloping plateaus and coastal plains. Only by way of the Hudson and Mohawk Valleys, Cumberland Gap, the Wachesa Trail,[undue weight? – discuss] and round about the southern termination of the system were there easy routes to the interior of the country, and these were long closed by powerful Native American tribes such as the Iroquois, Creek, and Cherokee, among others. Expansion was also blocked by the alliances the British Empire had forged with Native American tribes, the proximity of the Spanish colonies in the south and French activity throughout the interior.
Who had the Spanish colonies allied with?
{'text': 'Native American tribes', 'answer_start': 792}