The British and German Empires
At the beginning of the 20th century (1900) England was what the U.S.A. is today. England was an empire extending from Canada and East Africa, to India and Australia, covering most of the globe as you can see on the following rough map (green colour).
However after her victory against France in 1871, Germany established itself as one of the great powers, and with its rampant industry she became England"s main competitor. In a sense Germany was for England what China is for the U.S.A. today. The most important problem in the Anglo-German relations was Germany"s wish to extend her influence to the Persian Gulf through the Ottoman Empire.
Even though the shaky Ottoman Empire had lost most of her lands, she was still extending to the Persian Gulf through what today are Syria, Iraq and Saudi Arabia (parts of the green area on the above map). The alliance between the German Empire, the Empire of Austro-Hungary, Bulgaria and the Ottoman Empire, was a great threat for the English interests.
As you can see from my rough (red) diagram, the alliance between Germany, Austro-Hungary, Bulgaria and Ottoman Empire, would form a solid block extending from Germany to the Persian Gulf, since Syria and Iraq were Ottoman territories. A large part of the Balkans was also part of the Ottoman Empire at the time, and it was possible for the German Empire to connect to the Ottoman Empire and the Persian Gulf through Austro-Hungary, as you can see on the following 1912 map of the Balkans.
The Balkans in 1912
The above situation was a threat for the control of the Persian Gulf, which is the richest region in the world in terms of oil reserves, it was a threat for the control of the Caspian Region, which is the second richest region in the world in terms of oil reserves, and which was controlled by the Russians, it was a threat for India, which was England"s most important colony, and it would also bring the Germans very close to the Suez Canal and Egypt as you can see on the following map.
The Suez Canal and Egypt were under British Control, and were crucial for the control of India in South Asia, since the Suez Canal was cutting almost in half the distance between England and India.
The Suez Canal (black circle) was inaugurated a few decades earlier (1869).
