The utility of force changed profoundly during the second half of the 20th century. When battles were between gathered armies, as was the case up to, and including, the Second World War, it was possible to inflict a decisive defeat on an enemy – but different military calculations apply to what is known as ‘asymmetric warfare’, which has been defined as:
“leveraging inferior tactical or operational strength against the vulnerabilities of a superior opponent to achieve disproportionate effect with the aim of undermining the opponent’s will in order to achieve the asymmetric actor’s strategic objectives.” 
Since the Second World War, there have been several wars which have dragged on longer than expected. For example, George Bush's triumphalist "mission accomplished" speech after the invasion of Iraq was followed by a further 10 years of violence.
Guerrillas are likely to enjoy considerable success against military forces:
· Sophisticated aircraft and missiles, which were designed for conflicts between nations, are ineffective against guerrillas or insurgents who can easily hide among the civilian population.
· Guerrillas who use terrorist techniques, such as the ‘improvised explosive devices’ used in roadside bombs and suicide bombings, can inflict heavy losses on organised and visible troops without losing many of their own people’s lives – as has been amply demonstrated in Iraq and Afghanistan.
Adjustments therefore have to be made when assessing whether a war has a "reasonable chance of success".
© PatternsofPower.org, 2014
 Rupert Smith’s book The Utility of Force argues that military strategy has to be updated to meet new types of threat. The Times published a review on 18 September 2005, at the time of the book’s first publication, which was available in May 2014 at http://www.envirosecurity.org/ges/TheUtilityOfForceByGeneralSirRupertSmith.pdf
On 18 January 2007 The New York Times published another review, under the title Why the Strongest Armies May Lose the Newest Wars, which was available in May 2014 at http://www.nytimes.com/2007/01/18/books/18grim.html.
 Kenneth McKenzie, Jr., defined asymmetric warfare in “The Rise of Asymmetric Threats: Priorities for Defense Planning”, Chapter 3, Quadrennial Defense Review 2001, p. 2.
 As the BBC reported on 2 May 2008, in an article entitled No 'mission accomplished' in Iraq,
“President Bush did not say "Mission Accomplished" on the deck of the USS Abraham Lincoln off San Diego on 1 May five years ago. But the banner above him did.”
What the president did say was that major combat operations had ended (although many more people died in the following 10 years). This BBC report was available in May 2014 at http://news.bbc.co.uk/1/hi/7377036.stm.
A full transcript of the speech was available then at http://pix11.com/2013/04/30/transcript-of-president-george-w-bushs-mission-accomplished-speech/.
 The New York Times published an article entitled Improvised Explosive Devices, which was available in May 2014 at http://topics.nytimes.com/topics/reference/timestopics/subjects/i/improvised_explosive_devices/index.html. The article commented that “They [improvised explosive devices] have been the largest killer of American forces in Iraq and Afghanistan.”