This case was a dispute between the Western Union Telegraph Company and the Attorney General of the Commonwealth of Massachusetts. The Attorney General argued that the company was in violation of the state's laws by charging excessive rates for telegraph services. The Western Union Telegraph Company argued that the state laws were unconstitutional and that the company had the right to set its own rates. The Supreme Court ruled in favor of the Western Union Telegraph Company, finding that the state laws were unconstitutional. The Court held that the company had the right to set its own rates, as long as they were reasonable. The Court also held that the state laws were in violation of the company's right to due process and equal protection under the Fourteenth Amendment. The ruling in this case was significant because it established the principle that states cannot interfere with the right of companies to set their own rates. This ruling has been cited in numerous cases since then, and it has been used to protect the rights of companies to set their own rates.

In Western Union Telegraph Company v. The Attorney General of the Commonwealth of Massachusetts, the Supreme Court was asked to decide whether a state could impose taxes on telegraph companies operating within its borders. In a 5-4 decision, Justice Field delivered the dissenting opinion arguing that states have no authority to tax interstate commerce and that this particular tax imposed by Massachusetts violated both the Constitution and federal law. He argued that Congress had exclusive power over interstate commerce under Article I, Section 8 of the Constitution and any attempt by a state to regulate or interfere with it is unconstitutional. Furthermore, he noted that Congress had already passed legislation regulating telegraphs which preempted any action taken by individual states in regards to taxation or regulation of these services. As such, Justice Field concluded that this particular tax imposed by Massachusetts was invalid as it interfered with congressional powers granted under Article I and violated existing federal laws governing telegraphy services