Pohnpei397 is correct that it is difficult to prove why an attack did not occur, if an attack was actually planned. Unfortunately, he is unaware of dozens of terrorist attacks the prevention of which analysts have traced, at least in part, to the legal authorities granted federal agencies by the Patriot Act. Such cases of aborted terrorist attacks as those involving the so-called "Lackawanna Six," Yassin Aref and Mohammed Hossein, Hamid Hayat, Ahmed Omar Abu Ali, Assem Hammoud, and many others. In testimony before Congress in June 2013, senior officials from the Department of Justice and the FBI as well as the Office of the Director of National Intelligence and the Department of Defense stated that as many as 50 planned terrorist attacks had been thwarted. Some of these aborted attacks were the result of dumb luck on the part of law enforcement, but many were the result of improved cooperation among federal, state and local law enforcement agencies and between the FBI and the intelligence community.
As I noted in my original answer, the U.S.A. Patriot Act included a large number of provisions that directly addressed problems in the U.S. law enforcement and intelligence agencies that had contributed to our failure to prevent the attacks of 9/11. To suggest that it has been ineffectual is just plain wrong. Sometimes, you can explain why an event didn't occur, especially when you succeed in getting convictions on the people whose plans to carry out attacks failed to materialize.
It is very difficult to compare and contrast the benefits and problems associated with the Patriot Act in any objective way. In theory, the benefits and problems are quite clear. However, in practice, we do not really know what sort of impact the law has had.
The theoretical benefit of the Patriot Act is that it keeps us safer from terrorism than we would be without it. The Patriot Act was passed soon after the 9/11 terrorist attacks. It was meant to provide the government with more power to investigate potential terrorists. By giving the government these enhanced powers, we were supposed to be making ourselves safer from further terrorist attacks.
The theoretical problem of the Patriot Act is that it infringes on our civil liberties. Many of the enhanced powers that the act gives the government are, in the minds of some, excessive. People who are concerned with civil liberties believe that the Patriot Act allows the government too much of an ability to snoop in our private lives without a good cause to suspect us.
The problem with comparing the benefits and problems of this act is that we do not know whether the act has kept us safe and we cannot quantify how much our civil liberties have been infringed upon. There is no way for us to know if any legitimate terrorist plots have been prevented by government agents using the Patriot Act. On the other hand, there is no real way to say how much the government has been intruding on our lives.
Thus, we can clearly see the problems and benefits of the Patriot Act in theory, but we have a harder time seeing them in real life.
Immediately following the terrorist attacks of September 11, 2001, many members of Congress actively and forcefully sought legislative measures that would they believed would strengthen the ability of U.S. Government agencies to detect and prevent any such future attacks. The result of those efforts was the U.S.A. Patriot Act, officially titled the "Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act of 2001." Prior to passage of this Act, and partly as a result of the excesses among federal law enforcement and intelligence agencies that had occurred during the 1960s and early 1970s, strict prohibitions had been placed on the ability of these agencies to work together and to share information. The Federal Bureau of Investigation, in particular, was considered a domestic law enforcement agency that was expected, and required, to abide by federal statutes regarding the scope of its authorities. The Central Intelligence Agency, in contrast, was established in the late 1940s to collect and analyze information on foreign countries and global developments. Because of the aforementioned excesses, mostly involving investigations of the social reform and anti-war movements of the 1960s and 70s, Congress had passed laws designed to prevent the FBI and CIA from collaborating and sharing intelligence.
The terrorist attacks of 9/11 demonstrated the weaknesses inherent in the earlier structure. The invisible walls that existed between those two agencies prevented them from working together on investigations of al Qaeda, which had successfully “crossed the line” between existing as a foreign terrorist threat and a domestic criminal and terrorist threat. Because information gathered by one agency could not, under the law, be shared with the other agency, al Qaeda members who had entered the United States escaped detection by the FBI despite the CIA’s having tracked their movements. This is a simplification of the history, but should provide the basis for an academic understanding of the weaknesses that had existed in the U.S. prior to the 9/11 attacks.
Among the most significant changes made by the Patriot Act, then, was to remove the invisible bureaucratic walls between foreign intelligence and domestic law enforcement agencies, thereby enabling them to more efficiently share information. Another, and more controversial change made by the Patriot Act involved the authorities held by federal agencies to monitor and eavesdrop on individuals inside the U.S. At the time of the 9/11 attacks and throughout the drafting, debates and passage of the Patriot Act, this educator was employed a senior senator on the U.S. Senate Committee on the Judiciary. That senator had long wanted to change federal laws to allow for easier and more efficient monitoring of criminal and terrorist suspects inside the U.S. Because of his position on the Judiciary Committee, as well as being a member of the Senate’s leadership, he was able to have these changes included in the final bill that became the Patriot Act. In short, as occurred throughout the drafting of that new series of laws, members of Congress were able to use the crisis atmosphere following the 9/11 attacks to make statutory changes that in many cases they already had wanted to make, in some cases not without merit.
When discussing the strengths and weaknesses of the Patriot Act, then, perceptions are largely a product of where one stands on the issues of privacy and on the need to prevent the kinds of problems that occurred during the earlier decades mentioned above and that are occurring today with respect to reports of the National Security Agency’s ability to monitor electronic communications within and across international borders. The expansion of legal authorities in the Patriot Act for the use of so-called “trap and trace” devices, for example, which enable government officials to record the phone numbers of all outside calls made to a particular phone number inside the United States, has proven particularly controversial.
Among other controversial provisions in the Patriot Act are sections permitting the use of “roving” surveillance, in effect, the right to monitor communications and activities that are not confined to a single, specific location or instrument, such as a phone; greatly expanding the requirements placed on banks and other financial institutions with respect to customer activities that must be reported to the government (requirements designed to prevent or catch money laundering and the use of such institutions by terrorist organizations to raise and move money); and broadening the authorities of the FBI to use “National Security Letters,” which circumvent the normal court-authorized process for attaining search warrants by allowing the FBI to simply submit letters signed by their own officials to banks, Internet Service Providers, telephone companies, libraries and other institutions requiring those institutions to share the private records of specified individuals.
The strengths of the U.S.A. Patriot Act lie in the elimination of legal barriers to the sharing of information, the elimination of antiquated statutes pertaining to surveillance activities that were passed during earlier eras when communication technologies were more primitive, and the requirements placed on financial institutions to deter their tendency to turn a blind eye to suspicious activities. The weaknesses of the Act lie in the abuses the weakening of the Act’s authorities have facilitated by some government agencies, particularly, depending upon one’s perspective, the NSA.