At the time it was quite illegal. High treason, to be exact.
It also changed the world.
When the Signers pledged their lives in support of the Declaration they did so because they knew they could be hung for treason.
Indeed. Subversive of the established order, and radical by empowering the people over their government.
I think it was a once in humanity event, the results of which will be buried by those who write history.
It was no such thing.
The King and the British Parliament BROKE the English constitution by usurping rights which belonged to the King's subjects residing in the American provinces with respect to the taking of their property without their consent.
What if the president of the United States sent a team into your home to confiscate your weapons in violation of the American constitution - would you consider your resisting that to be treason?
What if the president came into your home and tried to evict you from your property without due process of law in violation of the American constitution - would you consider your resisting that to be treason?
What if the president tried that with respect to everyone in your state - would it be treason if they collectively declared the constitution in breach and reformed a new government?
The King and the parliament in London had no right under the British constitution (which, although not contained in a single document, was well understood) to take the property of the Americans without the consent of their representative assemblies.