Arthur Eddington became Professor of Astronomy at Cambridge in 1912. A pacifist, in 1916 when compulsory conscription was introduced he claimed exemption on grounds of conscience and received conditional exemption as long as he continued in his current job. Later this exemption was challenged. At the appeal in May 1918 Eddington said:

I am a conscientious objector. My objection to war is based on religious grounds. ... Even if the abstention of conscientious objectors were to make the difference between victory and defeat, we cannot truly benefit the nation by wilful disobedience to the divine will.'

The Astronomer Royal supported his exemption and suggested that Eddington undertake an expedition to observe the total eclipse the following year to test Einstein's General Theory of Relativity. The Tribunal granted him a twelve months' extension for him to do so.
During the war and amid hatred of all things German Eddington tried to keep bitterness out of astronomy and called for British scientists to maintain their pre-war friendships and working relationships with German scientists.

Becoming aware of Albert Einstein’s new General Theory of Relativity and as one of the few British astronomers with the mathematical skills to understand it, he became the chief supporter and expositor of relativity in Britain.

In 1919 he led an expedition (mandated by his Tribunal) to the island of Principe off the west coast of Africa, to observe the total solar eclipse of 29 May. His experimental results agreed with predictions made by Einstein, providing a verification of general relativity and leading to the theory becoming internationally famous.