When should the U. S.be at war?

You be the judge.

Mankind longs for peace and the means of escaping from the apocalyptic anguish brought on by the plague of war. (Making of America p 2)

What the U. S. Constitution says:

Article 1.8.11 The people of the states empower the Congress to declare war.

This provision gives Congress the exclusive RIGHT to declare war.

What the Founding Fathers said about war:

C. Pinckney: "Observed that the President's powers did not permit him to declare war."

Jefferson : "The question of declaring war is the function equally of both houses (of Congress)." "If Congress are to act on the question of war, they have a right to information (from the executive).
"We had reposed great confidence in that provision of the Constitution which requires two-thirds of the (Congress) to declare war. Yet it can be entirely eluded by a majority's taking such measures as will bring on war."
"The powers of declaring war being with the (Congress), the executive should do nothing necessarily committing them to decide for war."
"No country, perhaps, was ever so thoroughly against war as ours. These dispositions pervade every description of its citizens, whether in or out of office."

Franklin : "I would try anything, and bear anything that can be borne with safety to our just liberties, rather than engage in a war with such near relations (as the British), unless compelled to it by dire necessity in our own defense."
"At length we are in peace, God be praised, and long, very long, may it continue. All wars are follies, very expensive and very mischievous ones. When will mankind be convinced of this, and agree to settle their differences by arbitration? Were they to do it, even by the cast of a die, it would be better than by fighting and destroying each other."
"In my opinion, there never was a good war or a bad peace. What vast additions to the conveniences and comforts of living might mankind have acquired if the money spent in wars had been employed in works of public utility! What an extension of agriculture, even to the tops of our mountains; what rivers rendered navigable, aqueducts, new roads, and other public works, edifices, and improvements, rendering a...complete paradise, might have been obtained by spending those millions in doing good which in the last war have been spent in doing mischief; in bringing misery into thousands of families, and destroying the lives of so many thousands of working people, who might have performed the useful labor!"

Jefferson : "I do not believe war the most certain means of enforcing principles. Those peaceable coercions which are in the power of every nation, if undertaken in concert and in time of peace, are more likely to produce the desired effect." If nations go to war for every degree of injury, there would never be peace on earth."

Taken from the book, "Making of America" p 440 to 442

A Good Foreign policy is needed for Peace

The Monroe Doctrine contained a pledge which modern American leaders might well remember, and that is the promise that even though the United States would look upon any foreign invasion of the Western Hemisphere as a threat to her own security, she would not use her military power to interfere in the domestic or internal affairs of any other nation. Jefferson clearly stated, "Peace, commerce, and honest friendship with all nations, entangling alliances with none." The Founders' position on foreign relations is not "isolationism" but one of "separatism." They intended to become strong and independent, but not the policeman of the world. They warned against the tendency to favor one nation over another or to mix political interdependence with commercial and economic interdependence. They steadfastly opposed alliances which involved political interdependence. Their motto seems to have been summed up in the phrase, "Coordination yes, but consolidation no." Washington said. "Observe good faith and justice toward all nations. Cultivate peace and harmony with all." (The Constitution of the United States in a Nutshell p 27) (Making of America p 769)

A techniques used by revolutionists: A fervid but false solicitude for the unfortunate over whom they thus gain mastery and then enslave them." A quote by a wise man: David O. McKay

Is there any time when the President of the United States can declare war?
You can find the answer in the book, Making of America, The Substance and Meaning of the Constitution" by W. Cleon Skousen.