Here is the oath all Presidents (including Mr. Lincoln) must take before entering office:
''I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.''
I see a duty to "preserve, protect, and defend the Constitution of the United States", but I see no duty to preserve the Union. Mr. Lincoln ignored the Constitution. He did things that were clearly unconstitutional. He violated his oath of office.
Saying he did it to preserve the union is not an acceptable excuse. What right did he have to make war on the southern states? Clearly the right to secession is enshrined in the Declaration of Independence. If the 13 Colonies had the right to seceed from England, then the Confederate States had the right to seceed from the United States.
Since the states had the right to seceed, the employment of force against them was, in and of itself, illegitimate. Certainly Lincoln cannot use an illegitimate war as a reason to ignore the Constitution.
Article IV provides that "The Citizens of each State shall be entitled to all Privileges and Immunities of Citizens in the several states." Note also the Full Faith and Credit clause of Art. IV, Section 1. In light of these provisions, it was only by preserving every state's membership in the Union that President Lincoln would be able to "preserve, protect, and defend the Constitution of the United States".
Unionists in the Confederate states were being persecuted for their loyalties to the U.S. and denied by the Confederates the opportunity to vote in U.S. elections, conduct interstate commerce, have debts to them recognized in all states, travel between states freely, etc. Lincoln had a Constitutional duty to attempt to relieve them of this persecution and restore their privileges and immunities as U.S. citizens.
He did things that were clearly unconstitutional.
Name one (and be sure to cite a U.S. Supreme Court decision, since the Supreme Court is the highest Constitutionally designated judicial authority in the U.S.)
Clearly the right to secession is enshrined in the Declaration of Independence.
Lincoln most emphatically agreed that a right to revolution did exist, but persuasively argued that the Confederates' stated reason for seceding (i.e. to preserve slavery) was not a grievance which justifies revolution under the principles expressed in the Declaration.
Habeas Corpus is the writ that gets people in front of the court. By suspending Habeas Corpus, Lincoln made sure no cases would reach the court.
No, habeas corpus is the writ privilege which requires the government to obtain an indictment for an arrest. Anyone within the jurisdiction of the United States can obtain access to a federal court at any time, even when they are detained without an indictment.