The letters which were exchanged between the President and his premier set out the differences between them with the same distinctness.  Mr. Cass, after premising that he concurred with the general principles laid down in the message, says: 

  [Sidenote] Cass to Buchanan, Dec. 12, 1860.  Curtis, “Life of Buchanan,”
  Vol.  II., p. 397.

In some points which I deem of vital importance, it has been my misfortune to differ from you.  It has been my decided opinion, which for some time past I have urged at various meetings of the Cabinet, that additional troops should be sent to reenforce the forts in the harbor of Charleston, with a view to their better defense, should they be attacked, and that an armed vessel should likewise be ordered there, to aid, if necessary, in the defense, and also, should it be required, in the collection of the revenue; and it is yet my opinion that these measures should be adopted without the least delay.  I have likewise urged the expediency of immediately removing the custom-house at Charleston to one of the forts in the port, and of making arrangements for the collection of the duties there, by having a collector and other officers ready to act when necessary, so that when the office may become vacant the proper authority may be there to collect the duties on the part of the United States.  I continue to think that these arrangements should be immediately made.  While the right and the responsibility of deciding belong to you, it is very desirable that at this perilous juncture there should be, as far as possible, unanimity in your councils, with a view to safe and efficient action.

To this statement the President replied: 

  [Sidenote] Buchanan to Cass, Dec. 15, 1860.  Curtis, “Life of Buchanan,”
  Vol.  II, p. 398.

The question on which we unfortunately differ is that of ordering a detachment of the army and navy to Charleston, and is correctly stated in your letter of resignation.  I do not intend to argue this question.  Suffice it to say that your remarks upon the subject were heard by myself and the Cabinet, with all the respect due to your high position, your long experience, and your unblemished character; but they failed to convince us of the necessity and propriety, under existing circumstances, of adopting such a measure.  The Secretaries of War and of the Navy, through, whom the orders must have issued to reenforce the forts, did not concur in your views; and whilst the whole responsibility for the refusal rested upon myself, they were the members of the Cabinet more directly interested.  You may have judged correctly on this important question, and your opinion is entitled to grave consideration; but under my convictions of duty, and believing as I do that no present necessity exists for a resort to force for the protection of the public property, it was impossible for me to have risked a collision of arms in the harbor of Charleston, and thereby defeated the reasonable hope which I cherish of the final triumph, of the Constitution and of the Union.

  [Sidenote] Holt, conversation with J.G.N., 1874.