"Of all the enemies to public liberty war is, perhaps, the most to be dreaded, because it comprises and develops the germ of every other. War is the parent of armies; from these proceed debts and taxes; and armies, and debts, and taxes are the known instruments for bringing the many under the domination of the few. In war, too, the discretionary power of the Executive is extended; its influence in dealing out offices, honors, and emoluments is multiplied; and all the means of seducing the minds, are added to those of subduing the force, of the people. The same malignant aspect in republicanism may be traced in the inequality of fortunes, and the opportunities of fraud, growing out of a state of war, and in the degeneracy of manners and of morals engendered by both. No nation could preserve its freedom in the midst of continual warfare."
"Political Observations" (1795-04-20); also in Letters and Other Writings of James Madison (1865), Vol. IV, p. 491

"The constitution supposes, what the History of all Governments demonstrates, that the Executive is the branch of power most interested in war, & most prone to it. It has accordingly with studied care, vested the question of war in the Legislature. But the Doctrines lately advanced strike at the root of all these provisions, and will deposit the peace of the Country in that Department which the Constitution distrusts as most ready without cause to renounce it. For if the opinion of the President not the facts & proofs themselves are to sway the judgment of Congress, in declaring war, and if the President in the recess of Congress create a foreign mission, appoint the minister, & negociate a War Treaty, without the possibility of a check even from the Senate, untill the measures present alternatives overruling the freedom of its judgment; if again a Treaty when made obliges the Legislature to declare war contrary to its judgment, and in pursuance of the same doctrine, a law declaring war, imposes a like moral obligation, to grant the requisite supplies until it be formally repealed with the consent of the President & Senate, it is evident that the people are cheated out of the best ingredients in their Government, the safeguards of peace which is the greatest of their blessings."
Letter to Thomas Jefferson (1798-04-02); published in The Writings of James Madison
(1906) Edited by Gaillard Hunt, Vol. 6, pp. 312-14