Preparing for the Last War

Generals are always preparing for the last war. Of course, if it ran exactly the same way as next-to-last war, there would be no point to make such a distinction, so by induction we can guess the next one isn't going to be quite the same either. Which is why the connotation is that this way they are not quite catching the train. But, hey, doing things that way won us the last war, so we're going to do them that way again!

Of course, any expectations not backed by practice would be speculative. Thus as a rule even people who actively prepare are never quite ready for what happens the next.

Film

 * The Trade Federation in The Phantom Menace invaded Naboo by blockading it on a planetary scale while continuously landing infantry and assault tanks with the expectation that The Slow Walk of their Battledroids would eventually wear down the natives. While this doesn't work because they were using a Keystone Army, they tried this again during the Clone Wars; with the main difference using warship to protect their carriers in orbit while the ground forces marches towards the enemy with tougher droids and newer technology with mixed results.

Literature
""The one on the end's on rockers, sir; must be the officers.""
 * In the Belisarius Series, the Malwa regularly conquered by a combination of numbers and technology. When they met the Romans they met an enemy who was warned (by a messenger from the future) to develop their own technology. And a general who was not only a superb campaigner but capable of encouraging subversion through their empire.
 * In Honor Harrington, Haven regularly grabbed territory as a method of shoring up its revenue problems, creating a vast, if ramshackle, empire thereby. They found out when they met the Manticorans that they were years ahead in technology.
 * The Discworld novel Pyramids has a scene involving this trope: two nations that are obviously expies of ancient Greece and Troy go to war with each other, each using the weapon that won the last war. Thus, the war begins with a dozen wooden horses arrayed on each side of the line of battle.

Tabletop Games

 * During the "Interstellar Wars" era of the Traveller 'verse (a series of wars fought between the Terrans and the First Imperium between -2408 and -2219), the Vilani's theory of war was based on wars fought thousands of years before and had not changed. In between they had not been exactly peaceful, but most of their wars had been border wars or rebellions where they had overwhelming force, or rare succession wars fought against each other (and therefore not needing any improvement as like was fighting like). The Terrans were in a sense another border war. However not only did they have an ethos of innovation, their history of divided states encouraged them to make a science out of warfare. And they were soon far more advanced.

Web Comics

 * Schlock Mercenary had a moment with an uplifted ape [//www.schlockmercenary.com/2017-07-01 explaining] the concept to an alien new to the current galactic civilization.
 * Antihero for Hire backstory had a conflict called "The Unexpected War", in which Canada fought USA and annexed a few states… using genetically engineered dinosaurs because "the US was so busy protecting themselves against Weapons of Mass Destruction that they never made anything to protect against dinosaur attacks".

Real Life

 * The naval battles that took place during the Age of Sail revolved around both sides maintaining a line of battle that's sailing parallel to each other. While most of the seafaring European countries were using this by 1675; the main drawback is that with both sides lining up their warships, it usually creates a stalemate until one of them escapes or manages to surround a portion of the opposing fleet and tear it apar with their broadsides. And it's worth mentioning that the practice didn't really fall out of fashion until the end of the 18th century.
 * The reason for the notion's popularity was World War I, when Napoleonic style of warfare died because machineguns made an open massed advance suicidal, and artillery made field fortification necessary, so everyone had to adapt quickly.
 * …and then World War II came, and those who expected another World War I were introduced to the concepts of mobile warfare and air war.
 * There were quite a few complications, because sometimes Rock could beat laser for various reasons. For instance toward the end of 1942 carriers were all sunk or in for repairs and so surface naval battles returned. Except they were usually at night (because airplanes came out by day), and cruisers were the primary capital ship (because there were only so many battleships and those were gas guzzlers). And there never were any more Jutlands with dozens of battleships-but there was only one Jutland even in World War I.
 * Also every specialty with the guild-feuding that marked World War 2 had their own idea of what preparing for the next war would be. Even if they did intend to do such preparations. This could sometimes lead to unfortunate things like all the most glamorous specialties draining the infantry which no one wanted to get into for obvious reasons.
 * …except those who paid attention to Red October and wars in which the Soviet Union was entangled, including the Spanish Civil War: most high-ranked Red Army commanders participated in both last big wars, obviously considered their last and victorious big war, "the way to do it right", and hopelessly mired WWI abhorrent — and taught others this way. Which in turn led to promoting a very aggressive style that worked in highly mobile warfare, but was ill-prepared for adequate defence whenever "good offence" was not the best response.
 * Old-school eighteenth century officers in the Napoleonic Wars were not so much unprepared tactically (in fact they often thought themselves quite the forward thinkers). However there was a limit to how far their tactical speculation could go and it was limited to what a monarchist state or a Constitutional Monarchy (like Britain) or the few Medieval-style republics could manage. This usually meant a permanent army of otherwise unemployable soldiers commanded by traditionalists fighting temporary wars over territory. What it did not mean was the options that could be brought in by conscription. The new ideas included permanent corps and divisions (self-contained multi-armed sub-armies). As well as greater reliance on skirmishing tactics (which long existed but was not as emphasized). And greater trust in subordinate commanders. A lot of these things were speculated on for decades in clubs and military periodicals. But it took a Revolution in France to bring a government that would dare demand enough of its people to use these techniques.