We can make this argument by pointing out that it is very difficult to put any pressure on terrorists when they are not formally part of the government or the foreign policy of any country.
First, though, we must acknowledge that it is possible to put pressure on terrorists if a country has the technology and is willing to be ruthless. The US today has managed to put pressure on Al Qaeda through such things as its drone attacks. This pressure has in some ways made the war on terrorism successful.
However, it is hard to destroy the potential for terrorism because the terrorists are not a national force. Most of the 9/11 attackers were from Saudi Arabia, an important ally of the United States. The US cannot simply go into such a country, destroy its army, and be free from the threat. Instead, it must hope that its own efforts, combined with those of other countries, can reduce both the motivations for terrorists and their ability to act.
It may not be possible to say that a war on terror absolutely cannot be won in such circumstances, but it does make it much more difficult.