What part did Asia play in World War 1?

More than most people think, but not a major part of the outcome. Very little fighting took place in Asia, though the middle east saw fighting between the British and the Turkish Empires.

India was part of the British Empire and Indian troops served in France and Flanders and in Mesopotamia as well as other theatres of war. India was probably the Asian country with the largest involvement in terms of manpower. British India included modern India, Pakistan, Bangladesh, Burma and Nepal. Nepalese soldiers, called "Gurkhas" were particularly renowed for their ferocity and were regarded as some of Britain's best soldiers - in fact, they are to this day and Britain still maintains a Brigade of Gurkhas made up of Nepalese volunteers.

Japan was the most active independent Asian country. It was mainly interested in grabbing German colonies for itself. Japanese troops stormed the German fort of Tianjin in China, and took over some of the German Pacific Islands.

Thailand - or Siam as it was called at the time - declared war in 1917 in the hope that it would persuade the European nations to take it seriously as a country not as just the next subject for colonization. The Siamese actually sent a 1,200-man force to France, including a medical unit (it is claimed that the nurses there were the only women to serve in the trenches) and a fighter squadron. Some claim the fighter pilots were still in training when the war ended, others that they had just managed to fly a couple of missions before the armistice. 19 Thai soldiers were killed, and their names are on a war memorial in Bangkok.

Once it became obvious that the war was going to be won by the Allies, there was an unseemly scramble all around the world to declare war on Germany before it finished, in order to get a piece of the victory settlement.

China declared war on Germany but was a mess internally so couldn't spare any troops. Besides, the Western allies probably wouldn't have trusted Chinese soldiers in combat. Some Chinese labourers were sent to France and spent the last year of the war fixing roads and laying railways.

Vietnam, Laos and Cambodia were all part of the French Empire, and recent conquests at that. So the French couldn't rely on their soldiers. However some soldiers from this area (the French called it Annam) did go to France to work as labourers, like the Chinese.

The answer above regarding Vietnam, Laos and Cambodia was provided by an ignorant person. Vietnam's contribution to France was 21 bataillons serving both in the Western and Eastern fronts, in addition to 49,000 laborers. The streets of Saigon was named after numerous battles in which Vietnamese soldiers participated - Bar le Duc, Verdun, Avenue de la Somme, Quai de Belgique, Vosges etc.

Indochina provided half of the loans to France to finance the war as well as little less than 48% of all raw materials to support the war effort. Only West Africa provided more raw materials to France.