Extract the answer to the question from the following context. Question: Which of the following was not a serious rival for Britain: Napoleon, Russia or Siam? Context: In Britain's "imperial century", victory over Napoleon left Britain without any serious international rival, other than Russia in central Asia. Unchallenged at sea, Britain adopted the role of global policeman, a state of affairs later known as the Pax Britannica, and a foreign policy of "splendid isolation". Alongside the formal control it exerted over its own colonies, Britain's dominant position in world trade meant that it effectively controlled the economies of many nominally independent countries, such as China, Argentina and Siam, which has been generally characterized as "informal empire". Of note during this time was the Anglo-Zulu War, which was fought in 1879 between the British Empire and the Zulu Empire.
The answer to this question is:
Siam