Treason in 1787 was not an historical footnote. To most Americans, treason was synonymous with Tory, a/k/a Loyalist, those who were loyal to Great Britain. To the victor of the revolution went the spoils, and the sovereign States ruthlessly confiscated Tory property. Nine states went so far as to exile their Loyalists and five disfranchised them. Mandatory oaths of allegiance to States under penalty of loss of citizenship were common.
Who cared what the States did in this regard? Well, it had nation implications. The 1783 Treaty of Peace with Great Britain specified that no future confiscations could be made, nor any prosecutions could take place by virtue of any part a Loyalist played in the war. Repayment of prewar debt owed by Americans to British merchants was also required. In VA, future Governor Edmund Randolph declared that not even the resurrection of the prophets would convince Americans who owed money to English merchants that a mere treaty could force them to do so. Generally, the states responded they were not party to the Treaty and their judges could not enforce anything that was not State law.
Britains response was to retain control of the frontier forts they were supposed to surrender to the US. They fomented Indian attacks from these outposts along the Mississippi and Great Lakes against American settlers. Our weak government was getting Americans killed.
With this immediate background in mind, and knowledge of how treason could be used to silence political opponents as was occasionally done throughout English history, the Framers had to carefully craft the crime of Treason. In the space of an afternoon, they came up with the familiar language in Article III.
By defining treason, taking it and punishment for it away from the States, combined with the Supremacy of Treaties in Article VI, the US Constitution nullified relevant State laws. English merchants could sue for debt in state courts; British soldiers relinquished the frontier forts, and Americans began their westward settlement.
As well they should have. The United States did not - could not - fulfill the provisions of the treaty. A response was necessary.
They fomented Indian attacks from these outposts along the Mississippi and Great Lakes against American settlers. Our weak government was getting Americans killed.
Who was getting Americans killed? It was the behavior of the Brits and the tribes that was killing Americans. In pursuing the murder and terrorism of Americans, the British response went well beyond what was necessary to address the failure of the United States to fulfill all the provisions of their treaty. Nor did it end there. In a hissy snit of pique over the loss of their greatest looting target, Great Britain continued their harrasment to well near the end of the 19th Century,
British soldiers relinquished the frontier forts, and Americans began their westward settlement.
Its too bad that the relinquishment by the Brits did not include the instructions they received, purportedly, from William Wallace some 500 years earlier.