The Public Service Commission of Missouri et al. v. Brashear Freight Lines, Inc. et al., 1940 case involved a dispute over the regulation of interstate commerce by state authorities versus federal jurisdiction in such matters. The Public Service Commission of Missouri had denied an application from Brashear Freight Lines to operate as a common carrier within the state, arguing that it was protecting local businesses from competition and ensuring public safety on highways. However, Brashear argued this violated their rights under the Federal Motor Carrier Act which allowed them to engage in interstate commerce without interference from individual states' regulations or restrictions. The U.S Supreme Court sided with Brashear Freight Lines, ruling that while states have some authority to regulate commercial activities within their borders for reasons like public safety or fair business practices, they cannot interfere with federally-protected rights related to interstate commerce granted by Congress through legislation like the Federal Motor Carrier Act.

In the dissenting opinion for Public Service Commission of Missouri et al. v. Brashear Freight Lines, Inc., Justice Black disagreed with the majority's ruling that a state could not regulate interstate commerce rates even when such commerce had significant local aspects and impacts. He argued that this interpretation was too rigid and did not consider the realities of modern transportation networks where intrastate and interstate activities were often intertwined. Furthermore, he contended that states should have some authority to protect their citizens from potential harm caused by unfair pricing in these complex systems. The justice also expressed concern about federal overreach into areas traditionally controlled by states, warning against an overly broad application of the Commerce Clause which might undermine state sovereignty.