One of the main obstacles to the development of Iran's foreign trade over the centuries has been the location of its fertile northern provinces, which are shut off from the world's open seas by vast deserts and huge mountain chains. Since these provinces produced the country's most valuable export—silk—and since, after the transfer of the capital to Tehran, they included its largest consuming centers, trade had to be conducted over long caravan routes. In the seventeenth century, much silk was shipped through the Persian Gulf ports, but with the end of the Turco-Persian wars the bulk of Iran's westward trade passed across the Ottoman Empire—through Baghdad, and thence to either the Persian Gulf or the Mediterranean, or through Erzurum (Erzerum), and thence to Istanbul (Constantinople), Izmir (Smyrna) or Aleppo. However, all these routes were long, insecure, and expensive.