In the United States v. Gary Locke, Governor of Washington et al., 1999 case, the Supreme Court ruled that certain regulations imposed by the state of Washington on oil tankers were preempted by federal law and thus invalid. The State had enacted laws following a major oil spill in Alaska with an aim to protect its waters from similar accidents. These included requirements for tug escorts, navigational equipment, training and English proficiency for crew members among others which applied to both domestic and foreign vessels carrying crude oil across Puget Sound. However, these rules conflicted with federal laws governing maritime commerce as well as international treaties signed by the U.S., leading to this lawsuit being filed against them. The court held that while states have legitimate interests in protecting their coastlines from environmental harm caused by tanker accidents, they cannot unilaterally impose standards inconsistent with those set at national level or interfere with Congress's power over interstate commerce or executive branch's authority over foreign affairs.

In the dissenting opinion for United States v. Gary Locke, Governor of Washington et al., Justice Thomas argued that the majority's decision to invalidate Washington State's regulations on oil tanker design and operation was incorrect. He believed that these regulations did not interfere with federal law but rather complemented it by providing additional safety measures against oil spills in Puget Sound. According to him, Congress had not intended to preempt all state laws related to vessel safety when they passed the Port and Waterways Safety Act (PWSA). Instead, he contended that Congress only sought to establish a minimum national standard which states could supplement as long as their rules were not inconsistent with federal ones. Thus, he disagreed with the majority’s interpretation of congressional intent behind PWSA and felt this ruling undermined states' rights.