Border issues in the far south between the two countries were a constant concern for both, as Abyssinian leaders used to cross the Sudanese border in an attempt to annex it to Menelik's authority. British leaders continuously raised complaints through the British mission in Addis Ababa. Despite the shift of the Abyssinian state center towards the center in "Shewa" at the end of the nineteenth century, trade with Sudan across the western border did not diminish, especially after the vast lands rich in agricultural products and livestock that Menelik's expansions to the south and west provided. The stability that prevailed in Sudan to a large extent also helped develop this trade. In 1911, the volume of mutual trade reached £266,000, which led to a dispute between the two countries regarding the taxes imposed on exchanged goods. Qambela was then the main crossing for Abyssinian coffee towards Sudan before part of this trade shifted to the Karmak point, which received coffee from Abyssinian traders and sold them salt and cotton fabrics. The expansion led to an increase in Emperor Menelik's income after he added a large number of rich provinces with their agricultural and animal products to his authority, in addition to his monopoly on the ivory trade. The expansion also increased the slave trade, to the extent that the new southern and western provinces were described as "suitable for hunting humans and animals alike." The slave trade reached a level where emperors and provincial governors paid their expenses and gifts with slaves. Serious steps to combat this trade did not begin until after Abyssinia's accession to the League of Nations and after the intensive campaign launched by the British press against the slave trade in Abyssinia. To exploit European anger over the continuation of the slave trade, Italy claimed in its invasion of Abyssinia in 1936 that it wanted to stop the slave trade and slave-hunting raids. With the transfer of the capital to Addis Ababa, the commercial center of the empire also moved there, and the importance of Gondar and Metemma in the west weakened. Most traders headed east towards Zeila, Berbera, and Djibouti, especially after the construction of the railway line from Djibouti towards Addis Ababa, passing through Dire Dawa, where trade via this route reached 75%. This was followed by the northern route towards Eritrea, then the west towards Sudan; and finally, trade across the eastern borders with Somalia. The major traders were foreigners (Italians, British, Indians, and Greeks), as were the major princes.