I believe this is it:
To Braxton Bragg
Unofficial
Montgomery Ala. April 3 1861MY DEAR GENL.
"The Secty of War communicated to you last night by telegraph our latest information and the suppositions derived from it. It is, there is much reason to believe, with a view to exhibit power and relieve the effect of the necessary abandonment of Sumter that it is proposed to reinforce Pickens, but it is also possible that it may be intended to attempt the reinforcement of both. They will it is said avoid collision with you by landing their forces upon the Island and after the garrison is sufficient will bring in supplies defying your batteries."
"You will not have failed to notice that the tone of the Northern press indicates a desire to prove a military necessity for the abandonment of both Sumter & Pickens."
"It is already asserted that your batteries will not permit the landing of supplies, and soon this may be charged among the short comings of Mr. Buchannan, Per contra there is manifested a desire to show what can be done at Pensacola as proof of what would have been done at Charleston. In the latter view they may seek to throw both men and supplies into Pickens by landing on Santa Rosa beyond the range of your guns. It is scarcely to be doubted that for political reasons the U. S. Govt, will avoid making an attack so long as the hope of retaining the border states remains. There would be to us an advantage in so placing them that an attack by them would be a necessity, but when we are ready to relieve our territory and jurisdiction of the presence of a foreign garrison that advantage is overbalanced by other considerations. The case of Pensacola then is reduced the more palpable elements of a military problem and your measures may without disturbing views be directed to the capture of Fort Pickens and the defence of the harbor. You will soon have I hope a force sufficient to occupy all the points necessary for that end. As many additional troops as may be required can be promptly furnished."
"Instruction, organization and discipline must proceed with active operations; you will appreciate the circumstances which rendered such objectionable combination an unavoidable condition. Your batteries on the main shore are I am informed nearly complete and their converging fire may I hope compensate somewhat for their too distant location from the work to be battered. To secure the time necessary for you to effect a breach will it not be necessary to embarrass the use of the guns of Pickens which bear upon your works? Can this be done by a mortar battery placed on the Island so as to take those guns in reverse? In the same connection: could you establish gun batteries on the Island so as to drive off the shipping and prevent a junction of the land and naval forces? A mortar battery could I suppose be established in a night secure from fire & from sortie when you have a sufficient force to justify a partition of your army. If that first step /the establishment of a mortar battery/ was permitted you could establish your batteries also and then carry forward your approaches until you were attacked Then all your batteries being opened shells falling in the Fort from front and from rear must prove rapidly destructive to the garrison and open to you several modes of success - 1st By surrender - Second By abandonment if you had not been able to command the shipping Third By breach of front wall or explosion of glacis mine, exposing the work to capture by assault Fourth By evacuation on the plea that the means at the disposal of their government had not been sufficient to prevent the investment of the Fort and its reduction by famine."
"I have written to you freely and hurriedly because I wished to exchange views with you and felt assured that you would understand that there was no purpose to dictate; and under an entire confidence that your judgement would control your conduct, and could only be influenced by a suggestion, in so far as it might excite a train of thought out of the channel which the constant contemplation of a particular view is apt to wear. Though you are addressed <-as a Genl,-> upon /official matters yet I wish/ you to regard this not as a letter of the President but of your old comrade in arms, who hopes much, and expects much for you, and from you. very Respectfully & truly yr's. JEFFN, DAVIS"
[Lynda Lasswell Crist, "The Papers of Jefferson Davis Vol 7, 1861." Louisiana State University Press, 1992, pp.85-86]
James McPherson interpreted that letter in this manner:
"Jefferson Davis would have been quite happy if Seward had succeeded in his efforts to get the garrison out of Sumter. But he repudiated any notion that this gesture might lead to reunion; on the contrary, he would have seen it as a recognition of Confederate sovereignty. That is how Abraham Lincoln saw it too. Fort Sumter had become the symbol of competing claims of sovereignty. So long as the American flag flew over the fort, the Confederate claim to be an independent nation was invalid. The same was true of Fort Pickens, and Davis instructed General Bragg to prepare to attack it if and when an actual attack order came." [7]
"But no such order ever went to Bragg. The standoff at Sumter eclipsed the situation at Pensacola in the eyes of both Northerners and Southerners. When Lincoln informed South Carolina governor Francis Pickens of his intent to resupply the garrison at Fort Sumter, he forced Davis's hand."
7. Davis to Bragg, Apr. 3, 1861, Crist, PJD, 7:85–86.
[James M. McPherson, "Embattled Rebel: Jefferson Davis as Commander in Chief." Penguin Press, 2014, p.24]
Mr. Kalamata
"Let's also remember that on April 3 -- before Lincoln ordered the mission -- Jefferson Davis wrote Bragg to say he intended to start war at Forts Sumter and Pickens regardless of what Lincoln did or didn't do."
Perhaps BroJoeK has a different letter in mind. One in which Jefferson Davis writes that he "intended to start a war at Forts Sumter and Pickens regardless of what Lincoln did or didn't do."
Because a letter that said such a thing would lend credibility to his claim that Davis intended to start a war as a means of getting the other states to join in secession.