The original IRA helped win independence for most of Ireland in the early 1920s. But some elements in the movement refused to accept the settlement because it left Britain in control of Northern Ireland. This led to civil war in the new Republic of Ireland and a continuing series of on-again, off-again terrorist campaigns by splinter factions, usually against British interests in Northern Ireland (fueled by the resentment of Catholics to blatantly discriminatory treatment) and occasionally in the Republic, which officially opposes the use of terror. And Americans may perhaps recall events such as the 18th century "Shay's Rebellion" or "The Whiskey Rebellion," in which some of the "Patriots" needed to be reminded that the American Revolution was not against the idea of government but against non-participatory government. There still remain armed anti-government groups in the United States and most countries. But these groups usually, out of a sense of self-preservation, refrain from terrorism attacks.
Spain has a similar terrorist problem with its Basque minority and the Basque separatist movement ETA. A ceasefire was worked out in 2010 and a peace deal in 2011. Nearly half a century of ETA violence, and little to show for it, forced most ETA factions to try peace. This was forced on the ETA leadership by a growing loss in support from the Basque people. An example of this was the bombing at Madrid's Barajas Airport six years ago, which killed two people and injured about 20. It was the first ETA bombing in more than three years and came nine months after negotiations led ETA to declaring a "cease fire" in its forty-year for the independence of the Basque provinces of northern Spain.
Many groups, including many Basques across Spain, condemned the attack and the Spanish government declared that the "peace process" was finished, and there were calls for a more aggressive approach to eliminating ETA. But leading spokesmen for Batasuna, the Basque separatist party that is generally regarded as ETA's political wing, distanced themselves from the attack. In fact, the attack was uncharacteristic of ETA's past activities. Historically ETA attacks have tended to be focused on politicians, military personnel, and police officers and officials, with very low risk to "civilians." The attack in Madrid was different. This indicated a rift in the Basque nationalist movement. And that is nothing new in the history of insurgencies and resistance movements. Most ETA members opted for a peace deal but there are still militant ETA factions out there.
Success by an insurgency is usually measured by some sort of compromise. This often leaves some of the more committed rebels unhappy, for a variety of reasons. Perhaps they "want it all," everything the "revolution" was fighting for, even if it wasn't actually fighting for it. For example, the Algerian Revolution against France in the '50s was a nationalist movement but some of the nationalists thought of it as a religious war (to make Algeria an Islamic state, not a Western style republic), which has led to protracted religious insurgency in Algeria. And some of the rebels are often motivated more by a hankering for killing people and blowing things up than any ideological goal.
In the past, the fringe elements of movements like the IRA or ETA could continue the fight for decades. Under continuous secret service pressure such groups would tend to splinter into smaller and smaller fragments, over ideology or as popular support waxed or waned, or as their leaders cut deals to retire from the fight. As the history of the various manifestations of the IRA demonstrates, such "wars" can drag on indefinitely, characterized by sporadic incidents and usually low casualties rates.
There are important lessons here for the "Global War on Terror." Anyone can be a terrorist. Defeating the "leadership" will not necessarily end the conflict. In fact, there doesn't have to be a "leadership" at all. Anyone with an alleged-grievance or a "vision" can undertake his own "war." This sort of “one man movement” has happened in the past. New York City had "Mad Bomber" George Metesky in the '40s and '50s. The "American Republican Army" undertook two bombings in 1961. The "Unabomber", Theodore J. Kaczynski, was active from the 1970s to the '90s. These were actions of "super-empowered individuals," people with grievances, access to explosives, and enough skill to plant bombs without getting caught. Fortunately such crazed but capable people are rare. Terrorist movements attract a lot of the less-educated, less-skilled, and less-successful (and more easily caught).
But there's a difference between the past and the present. The world is now characterized by the near-instantaneous proliferation of information and misinformation, easy-to-use communication systems, and technologies that provide cheap, readily improvised WMD capabilities. At the same time, the development of our cultural, social, economic, industrial, and political structures offers vulnerabilities never dreamed of by earlier terrorists. This presents unprecedented problems for security forces, problems that are neither purely military nor purely law enforcement but a mixture of both, with a lot of complex intelligence demands. All this places complex strains on governmental jurisdictions and the intersection of the public and private sectors, not to mention civil liberties, cultural traditions, and privacy.