Treaty of Guarantee (proposed)
The Treaty of Guarantee was an agreement in which Britain and the United States guaranteed the French frontier against German aggression.  It came out of a proposal by Lloyd George, the British Prime Minister, at the Paris Peace Conference in 1919, following World War I, as a compromise to Marshal Ferdinand Foch's insistence that the Franco-German border be pushed back to the Rhine. Foch felt that the new border would prevent another German invasion into France. (France had been invaded from across the Rhine five times within a century: in 1814, 1815, 1870, 1914, and 1918.)

Origins at Versailles
Along with Foch, French premier Georges Clemenceau had demanded for Germany's Western border to be fixed at the Rhine.  Clemenceau relented when the Treaty of Guarantee was proposed, but Foch insisted that the French occupation of the Rhineland was crucial to halting future German aggression.

Lloyd George's proposal and Foch's protest
Lloyd George suggested a compromise.  If France relinquished its claims on the Rhine, Britain and the United States would guarantee France's boundary against future German aggression. Wilson agreed and treaties to that effect were drawn up.

If we do not hold the Rhine permanently [Foch told them] there is no neutralization, no disarmament, no written clause of any nature, which can prevent Germany from breaking out across it and gaining the upper hand. No aid could arrive in time from England or America to save France from complete defeat.

Rejection by Britain and United States
In return for abandoning the Rhine, Clemenceau accepted solemn guarantees of his country's frontier from his two great allies.  Both houses of the British parliament approved the Treaty of Guarantee in July 1919 if the United States also ratified it. The US Senate refused to approve it or the Versailles Treaty. That nullified the British assent. 

Clemenceau had been promised that aid, in return for giving up the security of the Rhine, which his generals had demanded. It was believed that Germany would not have risked invading France again if German rulers and generals had known in advance that Britain and the US would oppose it by military force.

That rejection made Clemenceau unpopular and ended his political career.