The attack on Sumter hardly justified a full-scale invasion. The attack, though idiotic and foolish in the extreme, was bloodless. Not a single federal soldier was killed during the bombardment, and the federal soldiers in the fort were given full military honors and then allowed to return home in peace. Moreover, the Confederacy offered to pay compensation for all federal installations in the South and to pay the South's share of the national debt.
After the Sumter incident, the Confederacy took no hostile action against the North. The wounding and dying did not start until a federal army invaded Virginia in July, three months after the bloodless Sumter assault. Under the Constitution, no federal force could intervene in a state without the permission of the state's legislature or governor, so if one insists on claiming Virginia was still in the Union, then the federal incursion was illegal.
The British were far slower to resort to force than the Republicans were. When the Patriots burned the HMS Gaspee in 1772, the British did not respond with a massive invasion.
In September 1776, even after four battles had been fought, including the bloody Battle of Bunker Hill, the British were willing to meet with Patriot leaders and offered full pardons and several major concessions in exchange for a cessation of hostilities and a resumption of British rule.
Amazingly, in 1778, after many battles and much loss of life, the British offered the Colonies representation in Parliament, the repeal all of the punitive acts of Parliament against the Colonies, and a form of self-rule that would include exemption from British taxation, if the Colonies would cease fighting and would recognize British rule. This was virtually everything the Patriots wanted. It would have granted the Colonies functional autonomy in almost every meaningful regard.