The Congressional power is known as the Supremacy clause.
In Article 6 Clause 2 of the United States Constitution, it states that federal laws are the supreme laws of the land. This means federal laws trump all other laws in the United States. So if the federal government wants to prosecute people for the possession of marijuana in a state where it is legal, they can legally do so thanks to the supremacy clause.
I think a desire to rebuild the country quickly and without ill feelings following the war.
False, that would be a primary source