Senate Rule 22, the rule which establishes the parameters for ending filibusters in the Senate, dates back to World War I and the threat presented by Imperial Germany's resumption of unrestricted submarine warfare in early 1917.
|5 March 1917 Headline|
After a Senate Filibuster Had Killed the Bill
The rule was adopted after 11 Senators ("a little group of willful men" in Woodrow Wilson's words) took advantage of the traditional rule of unlimited debate to prevent a vote on a bill to authorize arming of American merchant vessels as a deterrent to German U-boat attacks. The House had already passed the bill by a comfortable margin of 403 to 14). The arming of American merchant vessels proceeded apace despite the failure of this legislation when Secretary of State Robert Lansing advised Wilson that he had the executive authority to take action without congressional approval.
However, the president, sensing increased opposition to his foreign policy, encouraged limits be placed on Senatorial debate in the future. Later in the spring of 1917, the newly elected Senate adopted a rule providing that a two-thirds vote was needed to overcome a filibuster. In 1975 the Senate modified the cloture requirement, lowering the number of senators needed to end debate to 3/5ths of those “duly chosen and sworn” (60 under normal conditions).
Source: The Rule22 Blog