I assume you mean to say Congress.
But of course Congressional approval is not required for the President to move troops between US facilities.
For it to be an act of war it would have required the president to agree that SC had actually seceded and was now (part of) another country.
Which of course neither Buchanan nor Lincoln ever conceded.
Your argument that it was an act of war is therefore a classic example of a well-known logical fallacy.
http://en.wikipedia.org/wiki/Begging_the_question
Yay!
Wack a mole winner!
http://en.wikipedia.org/wiki/Begging_the_question
If I understand the term correctly, a lot of that goes on. Some people assume that everything that the secessionists or the Confederates did in the run-up to war was legitimate and put the blame on Lincoln. The questions they set up -- what they ask about and what they don't ask about, what they consider debateable and what they consider beyond debate -- determines they answers they get.
Rather than asking why the federal government or Lincoln behaved as they did and assuming that the secessionists or Confederates did nothing wrong or questionable or couldn't help acting as they did, I'd turn it around. It was possible that the federal government would take military action against secession. It was likely that if fired upon, the US would fire back and take the conflict to the next level. Given that -- knowing that -- why did the secessionist or Confederate leaders (who certainly did have various options available to them) behave as they did?