Why USA is called the United States and not just America? What is the difference between the Indian Constitution and the American Constitution?

Before getting into the topic one should know the meaning of the words Federal & Union.

Federal is an adjective which means having or relating to a system of government in which several states form a unity but remain independent in internal affairs.

Union, on the other hand, means the action of joining together or the fact of being joined together, especially in a political context.

To put it simply, the constitution which was made by Dr BR Ambedkar was for both Central government and the states. On the other hand, the Constitution made by Thomas Jefferson was only for the central government, this is why each state in America has its own written constitution.

In the case of India:

• Union/Central government has more control over the states.
• States must exercise their executive power in compliance with the laws made by the Central government(Article 356).
• No state government can impede on the executive power of the Central government within the states(Article 357).
• The Central government has the power to take over the state in matters related to national security( Article 352 to 360).

This is the reason why it is called 'a union of states'.

In the case of the USA:

• States have their own written constitution and are not controlled by the centre directly. For example, some states now allow the legalization of marijuana, both for personal and medical purposes. But federal law bans marijuana as a controlled substance.
• Under the Tenth Amendment to the U.S. Constitution, all powers not granted to the federal government are reserved for the states and the people.

This is the reason why it is called ' The united states''.

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Wrapping up:
 Try to understand this whole scenario by taking the example of Jammu & Kashmir. Before 31st October 2019, although it was an Indian state, still it had a separate Constitution, National Anthem, Flag.
Similarly, the states of America have their own constitution which means they are made to unite but actually have considerable self-rule.