I think I understand and agree with your first statement: the Federal Government would not be able to act like Lincoln did because it's now dominated by Liberals. Liberals believe every little group has a right to self-determination. They believe in Democracy. The support the rights of Palestianians for self-rule, etc. So if the people of Wyoming, by a significant margin, voted to leave the USA would liberals REALLY abandon ALL their beliefs, counter all their rhetoric, and FORCE poor Wyoming to stay in the Union? Would the same Senators voting for the Akaka bill to grant more autonomy and speical rights to Hawaiians (which is destined to end in Hawaiian independence) really support SENDING IN THE ARMY TO CASPER? I think not, I agree with you. Their own logic, emotion and oft stated beliefs will compell them to reluctantly admit Wyoming may leave the union.
As for your second point, it seems to have two point. While the Congress (dominated by leftists) might not want to fight a way to force states to remain in the union, the President might well want to. Then it would depend on the Army agreeing to do the fighting, possibly without legal approval from Congress.
What I don't understand is your focus on 'the national guard'. Wouldn't the regular Army and Marine Corp. be the ones to use?
There is the problem. Which side would the military be on? I think it would almost come down to individual soldiers, sailors, airmen and Marines. Each one may have a different "side" that they, as individuals, would support. Some may fire on civilians, some may not.
“What I don’t understand is your focus on ‘the national guard’. Wouldn’t the regular Army and Marine Corps be the ones to use?”
Wikipedia: The Posse Comitatus Act is a United States federal law passed on June 18, 1878, after the end of Reconstruction, with the intention of substantially limiting the powers of the federal government to use the military for law enforcement. The Act prohibits most members of the federal uniformed services (today the Army National Guard, Air Guard, and State National Guard forces when such are called into federal service) from exercising nominally state law enforcement, police, or peace officer powers that maintain “law and order” on non-federal property (states and their counties and municipal divisions) within the United States.
The statute generally prohibits federal military personnel and units of the National Guard under federal authority from acting in a law enforcement capacity within the United States, except where expressly authorized by the Constitution or Congress.
I think a rebellion, i.e., Secession, would see the Armed Forces quickly authorized by the Congress to be useable. At the present time, IMO, there would not be enough regular Army forces within U.S. borders to counter a Secession by multiple states. That could change after all the troops come home from Iraq; however, a conflict with Iran (which seems to be coming down the pike) would cause redeployment of the same forces.
To add to the problem of the U.S. military forces, the National Guard is currently highly mobilized to stations overseas, and so is also understrength as far as having to carry out operations within the U.S. boundaries.