Throughout 1969 and 1970, Cooper and Senator Frank Church co-sponsored the Cooper–Church Amendments, aimed at curbing further escalation of the Vietnam War. Congressional approval of one of these amendments on December 15, 1969, de-funded the use of U.S. troops in Laos and Thailand. Cooper had wanted to include a restriction on forces entering Cambodia as well, but Mike Mansfield, who helped Cooper write the amendment, feared that Cambodian Prince Norodom Sihanouk, who was officially neutral in the conflict, might be offended. When Sihanouk was deposed in 1970, Cambodia's new leader, Lon Nol, appealed to President Nixon for help in stabilizing his rule. Nixon agreed to send troops to Cambodia, despite protests from Cooper and others that this violated his stated goal of de-escalation in the region. Cooper and Church then drafted another amendment to de-fund U.S. operations in Cambodia; after negotiations with Nixon that continued funding until July 1970 so that the troops already in the country could be evacuated, the amendment passed 58–37. The House of Representatives later stripped the amendment from the legislation to which it was attached, and it did not go into effect. The amendment was nevertheless hailed by "The Washington Post" as "the first time in our history that Congress has attempted to limit the deployment of American troops in the course of an ongoing war." The fight over the Cooper–Church Amendments took its toll on Cooper's health, and he was briefly hospitalized to regain his strength. In 1971, Church, Mansfield, and George Aiken convinced Cooper to help them write an amendment to end U.S. involvement in Southeast Asia altogether, but ultimately, the measure did not have the support to pass and was abandoned.