Mills County v. Railroad Companies was a case heard by the United States Supreme Court in 1883. The case involved a dispute between Mills County, Iowa and several railroad companies over the taxation of railroad property. The county had assessed a tax on the railroads' property, but the railroads argued that the tax was unconstitutional. The Supreme Court held that the tax was unconstitutional because it violated the Due Process Clause of the Fourteenth Amendment. The Court reasoned that the tax was not based on a reasonable estimate of the value of the property, and that it was not applied uniformly to all railroad companies. The Court also held that the tax was not a valid exercise of the state's power to tax, as it was not based on a reasonable estimate of the value of the property. The Court's decision in Mills County v. Railroad Companies established that the Due Process Clause of the Fourteenth Amendment protects against arbitrary taxation. The Court's decision also established that the state must use a reasonable estimate of the value of the property when assessing taxes, and that the tax must be applied uniformly to all taxpayers.

In the case of Mills County v. Railroad Companies, the Supreme Court was tasked with deciding whether a county could tax railroad companies for their property located within its borders. The majority opinion found that counties do not have this power and thus denied Mills County's claim to collect taxes from these railroads. Justice Field dissented, arguing that states are allowed to grant taxing powers to local governments such as counties in order to raise revenue for public services like roads and schools. He argued that since there is no federal law prohibiting it, then states should be able to allow local governments such as counties the right to tax railroad companies if they so choose. In conclusion, Justice Field believed that allowing taxation by local government would benefit both citizens and businesses alike while also providing much needed funds for public services without burdening taxpayers too heavily or creating an undue burden on business owners who operate within those jurisdictions.