Want to stall global warming? Feed the poor? Prevent terrorism? If so, your first instinct probably wouldn’t be to ask a lumbering bureaucracy, dozens of countries, and thousands of political blocs to accept a single path to solve the world’s most urgent global crises. International treaties take years and, even on the outside chance they are ratified as originally conceived, have little guarantee of success. Yet the U.S. Senate now has the power to prove an exception to this maxim of international law. With one vote, it could enhance environmental stability and civil society across the planet. It is time for the United States to at long last ratify the U.N. Convention on the Law of the Sea.

The treaty, which establishes an international legal framework for governing the high seas, entered into force in 1994 and enjoys overwhelming global support. But until the United States ratifies it, the treaty is effectively a dead letter. The original objections that delayed ratification—largely the provisions on deep ocean mining—were resolved long ago, but ratification continues to be blocked by a handful of conservative Republican U.S. senators, including James Inhofe of Oklahoma, Jeff Sessions of Alabama, and Jon Kyl of Arizona, who are preoccupied by narrow and outdated notions of national sovereignty.

The issues that the convention would help advance are becoming steadily more critical, from the preservation of fish stocks, to environmental protection, and suppression of growing piracy and lawlessness on the high seas. A functioning treaty would bolster efforts to protect declining oceanic fish stocks, upon which nearly 15 percent of the world’s population depends as a primary source of protein. It would clarify access rights for strategic straits (ratification is supported by the Pentagon) and protect global commerce by...