There are many parallels between the steps taken by the United States since 9/11 and those that Abraham Lincoln took when confronted by our nation's greatest emergency and moment of national crisis. Just as today's terrorists take advantage of the freedom that America provides not only its own citizens, but foreigners as well, opponents of Lincoln's policies during the Civil War acted under "cover of 'liberty of speech,'" while their "spies… remain[ed] at large to help on their cause." In light of this crisis, Lincoln sought, as does the administration of George W. Bush, to achieve the proper balance between security and liberty. He acted carefully in taking what he believed to be the necessary domestic measures to prevent the collapse of the Union, and the end of republican government. Saving the Union required emergency measures that he nonetheless justified on constitutional grounds.

The debate over the proper balance between liberty and security goes back to the very founding of the American Republic. As James Madison wrote to Thomas Jefferson, "It is a melancholy reflection that liberty should be equally exposed to danger whether the government have too much or too little power." Lincoln addressed the dilemma in his July 4, 1861 message to a special session of Congress, in which he justified his emergency measures, including suspending the writ of habeas corpus, after Fort Sumter. "Is there," he asked, "in all republics, this inherent, and fatal weakness? 'Must a government, of necessity, be too strong for the liberties of its own people, or too weak to maintain its own existence?'"

Ultimately, the issue here is one of prudence. According to Aristotle, prudence is concerned with deliberating well about those things that can be other than they are (means). Prudence requires us to be able to adapt universal principles to particular circumstances in order to arrive at the means that are best given existing circumstances. In the case of the war against terrorism, the end is the survival of republican government. If we are to maintain this end, i.e. ensure the survival of the American Republic, what means are necessary and proper under the circumstances? What would prudence dictate regarding the balance between security and civil liberties in time of war?

In his excellent book, Republican Empire: Alexander Hamilton on War and Free Government, my good friend and Naval War College colleague Karl Walling has argued that throughout the history of the American Republic, a tension has existed between two virtues necessary to sustain republican government—vigilance and responsibility. Vigilance is the jealousy on the part of the people that constitutes a necessary check on those who hold power lest they abuse it. As Thomas Jefferson wrote, "it is jealousy and not confidence which prescribes limited constitutions, to bind those whom we are obliged to trust with power."

But while vigilance is a necessary virtue, it may, if unchecked, lead to an extremism that incapacitates a government, preventing it from carrying out even its most necessary and legitimate purposes, e.g. providing for the common defense. "Jealousy," wrote Alexander Hamilton, often infects the "noble enthusiasm for liberty" with "a spirit of narrow and illiberal distrust."

Responsibility is the statesmanlike virtue necessary to moderate the excesses of political jealousy, thereby permitting limited government to fulfill its purposes. Thus in Federalist 23, Hamilton wrote that those responsible for the nation's defense must be granted all of the powers necessary to achieve that end. Responsibility is the virtue necessary to govern and to preserve the republic from harm, both external and internal. For instance, the dangers of foreign and civil war taught Alexander Hamilton that liberty and power are not always adversaries. The "vigor of government is essential to the security of liberty."

Abraham Lincoln acted on this principle. Accordingly, both critics and advocates have argued that Lincoln essentially assumed dictatorial powers during the Civil War—that he took extra-constitutional steps to save the Union. But while the Constitution is silent on some of the steps he, took, it is possible to argue that everything he did was justifiable under the powers stipulated in Article 2 of the Constitution.

Lincoln's steps are well known. Under his constitutional powers as commander in chief of the military, he declared martial law and suspended the writ of habeas corpus in certain locations. He blockaded Southern ports. He shut down some opposition newspapers. He created tribunals similar to the ones that President Bush has established. At one point early in the war, convinced that the Maryland legislature was poised to vote an ordinance of secession, he ordered Federal troops to arrest and detain pro-secessionist lawmakers. Lincoln justified this last step on the grounds that there was "tangible and unmistakable evidence" of their "substantial and unmistakable complicity with those in armed rebellion."

Even Lincoln's supporters question the constitutionality of his actions. After the emergency had passed, the Supreme Court ruled in Ex Parte Milligan that citizens could not be tried by military courts outside of a war zone when civil courts were operating. And many contemporary historians believe his decision to suspend the Great Writ was improper.

But Lincoln was sailing in uncharted waters. The emergency he faced was unprecedented. Lincoln went out of his way to maintain constitutional forms. But those who would condemn him need to realize that the president has his own constitutional source of power: he is the Commander-in-Chief, which directly bestows upon him powers in times of military crisis that are not derivative of any congressional power. One of these powers is the prerogative.

According to John Locke, the prerogative is "the power [of the executive] to act according to discretion for the public good, without the prescription of the law and sometimes even against it." Since the fundamental law that the executive ultimately must implement is to preserve society, it is "fit that the laws themselves should in some cases give way to the executive power, or rather to this fundamental law of nature and government, viz. that as much as may be, all members of society are to be preserved."

The prerogative is rendered necessary by the fact that laws arising from legislative deliberation cannot foresee every exigency. For the safety of the republic, the executive must retain some latitude for action. Thus since Congress was not in session when Ft. Sumter was attacked, Lincoln took the steps he believed were necessary to preserve the Union, because inaction would have lost the game.

Of course, prudence dictates that the prerogative be exercised rarely, i.e. only during the most dire emergencies. Thus what is applicable during time of war is not appropriate during a time of peace. Lincoln's letter to Erastus Corning and a group of New York Democrats who had criticized his war measures is a study in the application of prudence.

He wrote "…I can no more be persuaded that the Government can constitutionally take no strong measures in time of rebellion, because it can be shown that the same could not lawfully be taken in time of peace, than I can be persuaded that a particular drug is not good medicine for a sick man, because it can be shown not to be good for a well one. Nor am I able to appreciate the danger apprehended by the meeting [of the New York Democrats] that the American people will, by means of military arrest during the Rebellion, lose the right of Public Discussion, the Liberty of Speech and the Press, the Law of Evidence, Trial by Jury, and Habeas Corpus, throughout the indefinite peaceful future, which I trust lies before them, any more than I am able to believe that a man could contract so strong an appetite for emetics during temporary illness as to persist in feeding upon them during the remainder of his healthful life."

Prudence dictates that in time of war, responsibility trumps vigilance. In response to criticism of his suspension of the writ of habeas corpus, Lincoln asked, "…are all the laws but one, to go unexecuted, and the government itself go to pieces, lest that one be violated?" Lincoln's point is as applicable today as it was during the Civil War. If those responsible for the preservation of the Republic are not permitted the measures to save it, there will be nothing left to be vigilant about.

Mackubin Thomas Owens is professor of strategy and force planning at the Naval War College in Newport, RI, and an adjunct fellow of the Ashbrook Center. The views expressed here are his own and do not reflect the position of the War College, Navy Department, or Department of Defense.