It wasn't a first resort. The South had tried to negotiate.
The delegation was sent to Washington "for the purpose of negotiating friendly relations between that government and the Confederate States of America, and for the settlement of all questions of disagreement between the two governments upon principles of right, justice, equity, and good faith." That's not negotiation, that's an ultimatum. Accept the legitimacy of the southern rebellion, period. Vague offer to settle 'disagreements' aside, unless Lincoln was willing to accept southren independence then there was nothing to talk about. Had everything been on the table, including reunification, then you might have a point but laying down a 'take it or leave it' proposition, and then using that as an excuse to claim wounded honor and start a war is nonsense.