Metre Convention
At an international commercial congress the Treaty of the Meter, also known as the Metre Convention ("Convention du Mètre") of 1875 was signed by 17 countries, including the US, making the metric system the international system of weights and measures. Note that this was a meeting of international states to facilitate commerce. This treaty, then, falls under the overview of the United States Department of State, which has devolved oversight to the United States Department of Commerce.