Yes, Lincoln broke laws. “Union never formally declared war.”
What do we call invasions conducted without a formal declaration of war?
Declare war on who? The United States? You declare war on other countries and not rebellious parts of your own.
But since you're on the subject, if it really considered itself a sovereign nation then shouldn't the Confederacy have declared war on the U.S. before attacking Fort Sumter?
We can say Lincoln broke no laws because: Congress eventually approved everything he did.
But more to my point: unlike Jefferson Davis, Lincoln broke no laws for the purpose of destroying the US Constitution.
Instead, all of Lincoln's efforts were to save the Union and its Constitution.
BJK: “Union never formally declared war.”
JCBreckenridge: "What do we call invasions conducted without a formal declaration of war?"
In all of history, formal declarations of war have only been used between nation-states, never to defeat rebellions.
That's why the Confederacy declared war on the United States -- because it wanted to show that it too was a nation-state.
The Union did not declare war because it considered the Confederacy a rebellion.