What does President Lincoln mean in his First Inaugural Address when he says the declared purpose of the Union is to constitutionally defend and maintain itself?

Answer 1

Secession from the union was not legal or possible

Answer 2

President Lincoln meant that the primary goal of the Union is to protect and preserve its constitutional framework. He believed that the Constitution was the foundation of the nation and it was the duty of the government to uphold and defend it. Lincoln's emphasis on the "declared purpose" suggests that maintaining the Union was a fundamental principle that should not be compromised.