There is no amendment in the US Constitution regarding State governments being no longer necessary. What the US Constitution does say is that whatever the US Constitutions laws do not cover, belong to the individual States to determine.
Twenty-sixth amendment
In unstable governments, conflicts are often decided by whomever is able to obtain the most power. Whoever holds the most power is able to crush/curtail the opposition and stay in power.
Jesus
When used correctly, the word "federal" (as in "federal system") refers to the division of political power between the national government and the state governments. However, people use the word "federal" as a synonym for the word "national." Therefore, when people use the phrase, "Today, the federal government decided...." they are (usually) referring to the national government and not the division of power between the national and state governments. This is an example of a technical term in political science being incorrectly used.
Only by amendment of the Constitution of the United States
The 18th amendment. It made drinking intoxicants illegal. After much failure in stopping people from drinking the government decided to end the law.
The Judicial Branch
The governments decided to introduce a gold licence so people won't bother coming because you had to pay to mine for gold
22nd amendment
The 7th amendment
The "Federal Government" or "National Government" known generally as "The United States of America" was seen as necessary to prevent human rights violations (a slide into the former governmental practices from England) at the time it was created.
The 18th Amendment (dealing with prohibition) was repealed by the 21st Amendment because of its lack of usefulness. People were still drinking, but the government couldn't benefit from this at all (taxes, anyone?) and so they eventually decided that it was better to just keep drinking alcohol legal but put more restrictions on it.
The government has done many things to save ozone. Banning CFC's, depleting CFC's etc.
The 18th Amendment enacted Prohibition. It proved to be highly unpopular, and was largely ignored by the population who continued to drink at speakeasies. The crime rate skyrocketed under Prohibition, as gangsters like Al Capone made fortunes bootlegging alcohol. The Government decided to cancel the 18th Amendment as it was ineffective and unpopular. The 21st Amendment repealed Prohibition.
Twenty-sixth amendment
In unstable governments, conflicts are often decided by whomever is able to obtain the most power. Whoever holds the most power is able to crush/curtail the opposition and stay in power.
22nd