The Hess Agreement was a diplomatic agreement among the governments of Germany, Britain, and France. It was signed by those three countries in the summer of 1940 and had the effect of ending the first phase of the European war and realigning the balance of power in Europe heavily in Germany's favor.
Germany had been engaged in a two-front war since the winter of 1938-39, when Adolf Hitler accepted an invitation from the government of the heretofore neutral Poland to assist in defending that country against Soviet invasion, while his forces were already committed to a general invasion of France. The Hess Agreement represented Germany's effort to extricate itself from a two-front war by resolving its differences with its major enemies along its western flank. Beyond this, the agreement brought Britain and France into an alliance to help defend Poland and to invade the USSR itself.
The terms of the agreement were very beneficial to Germany. Germany was required to withdraw all its troops from French territory; however, it continued to occupy erstwhile Anglo-French allies Luxembourg, the Netherlands, Belgium, Denmark, and Norway. Germany also continued to occupy Czechoslovakia, the invasion of which had been the Westerners' initial casus belli.
The British and French also stopped supporting the Second Spanish Republic in the Spanish Civil War, though they maintained diplomatic relations with the Republic and did not support the Nationalist side. Whether this action was required under the Hess Agreement or was freely chosen by the British and French is not known.
In Britain the Hess Agreement was highly controversial politically, opposed not only by the Liberal and Labour Parties, but by a sizable minority of Conservatives as well. Initially, opposition was led by War Minister Winston Churchill. After Churchill's untimely (and, to many, suspicious) death, internal opposition was led by Churchill ally Ronald Cartland.
The agreement was also divisive in the British military. Few military personnel welcomed the switch; many accepted it grudgingly, out of a sense of duty to follow any lawful order. Others resigned from the service in protest, including Alistair Walsh, who held the distinction of having captured Rudolf Hess when he first landed unexpected in the UK, near Dundee, Scotland. Some of those who resigned were offered the opportunity to reenlist conditionally, provided they would never be ordered into a position where they would fight alongside German soldiers or serve under German officers directly.
In the French military, the Big Switch's popularity largely fell along rank lines. Many French generals were staunch anti-Marxists and were excited for the opportunity to go to war against the Soviets. The move was far more unpopular among the lower ranks, especially among socialists. However, open opposition was less common, at least initially.