The Constitution defines "treason against the United States" as "...levying War against them, or in adhering to their Enemies..."
It does not say: those who commit treason are thereby no longer citizens.
So all presidents since George Washington enforced laws against treason, with or without prior Congressional approval.
We might also note that Confederate President Davis did not wait for Confederate Congress to formally declare war before starting war against United States Army troops in Fort Sumter.
JCBreckenridge: "What gave Lincoln the authority to fire on Fort Sumter?"
What are you talking about?
Lincoln didn't "fire on Fort Sumter".
That was Confederate President Jefferson Davis.
JCBreckenridge: "What gave Lincoln the authority to mobilize troops to fire on American citizens?
What gave him the authority to cross the Virginia border and invade?"
Might I suggest you study the Militia Act 1792 for starters?
Then please remember that Lincoln did not "cross the Virginia border" until after the Confederacy formally declared war on the United States, and Virginia formally joined the Confederacy's war.
“levying War against them, or in adhering to their Enemies”
Which South Carolina and the Confederacy did not do. Did the Confederacy invade the North? No.