TERRORISM

According to American Heritage Dictionary, terrorism refers to the unlawful use or threatened use of force or violence by a person or an organized group against people or property with the intention of intimidating or coercing societies or governments, often for ideological or political reasons

Britannica Concise Encyclopedia: defines terrorism as systematic use of violence to create a general climate of fear in a population and thereby to bring about a particular political objective. It has been used throughout history by political organizations of both the left and the right, by nationalist and ethnic groups, and by revolutionaries. Although usually thought of as a means of destabilizing or overthrowing existing political institutions, terror also has been employed by governments against their own people to suppress dissent; examples include the reigns of certain Roman emperors, the French Revolution, Nazi Germany, the Soviet Union under Stalin, and Argentina during the "dirty war" of the 1970s.

Reasons why it is increasingly difficult to root out terrorist networks and their activities across the world
Terrorism's impact has been magnified by the deadliness and technological sophistication of modern-day weapons and the capability of the media to disseminate news of such attacks instantaneously throughout the world. The deadliest terrorist attack ever occurred in the United States on Sept. 11, 2001 attacks, when members of al-Qaeda terrorist network hijacked four commercial airplanes and crashed two of them into the twin towers of the World Trade Center complex in New York City and one into the Pentagon building near Washington, D.C.; the fourth plane crashed near Pittsburgh, Pa. The crashes resulted in the collapse of much of the World Trade Center complex, the destruction of part of the southwest side of the Pentagon, and the deaths of some 3,000 people.
The first is because it is rooted in religion. This can cause people to act in ways that are very illogical or counter-intuitive (e.g. willingness to blow yourself up). Looking back at history, religious clashes have often been the most difficult to resolve and end because people feel so strongly about their beliefs. Because terrorists are under the belief that they are doing god's will, they will go to extremes beyond what is usually seen to accomplish their mission.
Another reason is the difference in cultures. While many of the local population is not activly involved in terrorist activities, they are much less likely to aid those fighting terrorism because those fighting terrorism are still seen as outsiders, where the terrorists come from the same religion, country, etc.

Lastly, it is because it is a non-conventional war. Almost guerrilla like tactics. These types of fights (whether it be the US fighting the British during the revolutionary war, or fighting in the jungles of Vietnam) are always more difficult. It is not a traditional battle (e.g. the US civil war, WWII, etc) where the two face-off, one side wins, the loser agrees to give up their fight, and the two sides move forward looking towards the future.
Terrorism operates against the basic rules of engagement Most notably - it operates against civilians, and sometimes from within civilian population.It strikes at random, which makes it harder to defend against.Its impact is not only the physical damage it causes but mostly the effect on day-to-day life.
Even defending against terror using conventional means contribute to its psychological effect. or these reasons conventional war-like methods are ineffective against terrorism.In an overly-"politically-correct" society, simple risk-assessment statistics-based deductions becomes "racial profiling", which makes life even harder.
Because we umm illegally kidnap suspected terrorists and keep them indefinitely without charge in torture camps (have tortured over 40 suspected terrorists to death), we occupy foreign sovereign nations and continue to undermine democracies for economic interests? Might be a start as to why it has increased sevenfold since 2001. I would think these things would encourage more terrorism and new followers of their 'hate'.
CONCLUSION
Just generally, why is terrorism, especially Islamic Fundamentalists, so hard to fight? Is it because of the invention of more advanced and sophisticated weapons? Is it because of tactics? Is it because nuclear missiles have placed a humongous threat on the fight on terrorism? Please help..