We can understand "curse" in several ways: as the invoking of evil or misfortune upon another, or as the evil or scourge itself. The proverb primarily deals with invoking a curse against another when no justification for doing so exists. Such a curse is akin to the aimless flitting of birds, suggesting that it will have no effect. It will not "hit" its intended target.
We can definitely consider the tragedies of September 11 as a curse. However, God undoubtedly approved of it, or it never would have happened. This curse hit, and it hit hard. Therefore, we must conclude that there was more than ample justification for it falling upon this nation. The death toll was approximately 3,000 people, a horrendous figure to be sure, but it pales when compared to just one other death-toll figure: Every day in the United States over 4,000 human lives are snuffed out of existence by abortion. In the 30 days following September 11, 120,000 lives ceased to exist. Of those 120,000 abortions, 95% of them—114,000—occurred solely for the mother's convenience!
Is it any wonder that God cries out in Ezekiel 7:23, "Make a chain, for the land is filled with crimes of blood, and the city is full of violence." George Mason, whose great influence can be seen in our Declaration of Independence and Constitution, wrote that sin "brings the judgment of heaven upon a country. . . . By an inevitable chain of causes and effects, Providence [God] punishes national sin by national calamities." George Washington declared in his First Inaugural Address, "We ought to be no less persuaded that the propitious smiles of Heaven can never be expected on a nation that disregards the eternal rules of order and right, which Heaven itself has ordained."
The apostle Paul states in Romans 9:14. "What shall we say then? Is there unrighteousness with God? Certainly not!" As a people, we are guilty and fully deserve anything He in His loving wisdom decides to inflict upon us. Most assuredly, we are not innocent victims. Individually, few of us have sinned against any of the people, groups, or nations that may have done this, but as citizens, we are part of this nation, and our well-being rises and falls with it. We have eagerly accepted God's overflowing abundance of material blessings with which He showered this nation. So when He judges that we need to be brought down a peg—or many pegs—we would do well to consider deeply the many ways we may have offended the great God who created us and gives us every breath of air we breathe.