Is there any basis for the claim that guerrilla forces are invincible? The British Empire has bested them in a few vicious wars – but at what cost? And for how long? The answers often lie beyond the battlefield

After Britain declared war on France in May 1803, Napoleon urged the Spanish Empire, his ally, to join the French troops in defeating his sworn enemy. At the end of 1807, though fatigued by decades of combat in the American War of Independence and on both sides of the French Revolutionary wars, the Spaniards helped Napoleon conquer Portugal. He then turned on them, invading Spain and adding it to the jewels in his European crown. Apart from his distrust of the Spanish, Napoleon had sought to replace their king with a puppet monarch – his brother Joseph. 

Following the French invasion, British forces came to Portugal’s aid, encouraging popular uprisings both there and in Spain. Hiding in forests and mountains, and assisted by locals, the rebels adopted a “hit and run” approach to Napoleon’s entrenched, superior forces. The Spanish dubbed this form of warfare “guerrilla” – “little war.” 

Carl von Clausewitz, a Prussian officer in the Napoleonic wars who later wrote the classic, multivolume work On War, adopted the term, defining it as small warfare. According to him, guerrilla warfare involves an irregular force resisting and harassing a regular army that controls the territory. Much of the action in such campaigns focuses on disrupting supply routes and attacking the often foreign ruler’s isolated units and garrisons.

In the Spanish uprising, the guerrilla tactics were intended to weaken the French forces, making them vulnerable to the concerted efforts of the Anglo-Portuguese-Spanish army commanded by Arthur Wellesley, better known as the first Duke of Wellington, a title he later received in 1814. His men eventually defeated Napoleon’s armies in a series of large-scale, pitched battles. 

As Vietnamese military leader Nguyen Giap explained in his book People’s War, People’s Army (1961), guerrilla warfare should either supplement a regular army’s offensive or allow rebels to build up a new force capable of defeating the enemy in the open, as Vietnamese insurgents did, overpowering the French at the battle of Dien Bien Phu in 1954. 

Yet not every military campaign employing hit-and-run tactics can be described as a guerrilla war. Such assaults on an advancing army can simply be an alternative form of defense, sometimes also known as screening, or mobile defense. One side attacks, retreats, and then reattacks elsewhere along an extended front line. Regular armies frequently utilize such strategies, shifting forces around to block the enemy and wear it down. Indeed, mobile defense played an important role in World War II wherever an extended front challenged its defender’s military capability of continuously protecting the entire line. This tactic remains part of regular warfare in the 21st century, as seen in Hezbollah’s use of mobile defense tactics against advancing Israeli forces during the Second Lebanon War (2006). 

On the other hand, in the current conflict in Gaza, the Israel Defense Forces (IDF) face typical guerrilla forces, with Hamas frequently launching small, surprise attacks on Israeli soldiers who’ve secured an area. Rather than block the Israeli advance, Hamas capitalizes on it: Israeli soldiers in ostensibly safe zones become targets for further guerrilla attacks either from the rear or from underground tunnels. The extensive subterranean network throughout the region was built precisely for this purpose, aiming to permanently mire the IDF in the Gaza Strip.