Why did trench warfare accept deadlocks?

So most people look to movies when they think of War but this is the greatest lie ever told. Most War is simply the act of nothing getting done and even less happening (The Front.) World War 1 in the West is the textbook example of this...with the Battle of Virginia during the US Civil War an excellent precursor. It was quite common in the latter for Johnny Reb and a Yank who would take up sniping positions in trees and other assorted locations as skirmishers to strike up a conversation with another...literally of the most friendliest of terms save the handshake.

This may sound odd for those brought up under the Manichean View of "Us versus the Terrorists" but Americans have a long History of yucking it up with the Enemy as our first Enemy claimed to be the very English we were fighting. So you can imagine all sorts of confusions as a result.

This could never be true in World War 1 given the language and cultural differences...but the line was still the same....sometimes mere yards from one another. I would recommend googling a comparative study of the actual trenches themselves. You'll find very quickly German and Austrian trench systems were far better built and "organized" than those in the West. Such an advantage cannot be overstated. Even the French Army mutinied over offensive activity in their own country against assaulting German and Austrian trench positions outside of Paris. The reason for the "system" proceeded by the logic of Germany having to fight on two fronts simultaneously. The West was too strong...the East was attacking..just dig in in the West and deal with the issues in the East as that was where the the "means of War" would be gathered for the Axis Powers.

The forces didn't go around the trenches, not only because the trenches pretty much extended across the entire front, but also because neither army had the ability to move large numbers of troops quickly. Most troop movements near the front were on foot, at about 2mph, or slower if attacking or moving across ground churned up by artillery.

Even if the attacker could break through the enemy's trenches or find a weak spot and overrun it, that slow moving attacking army ran into even more of:

The machine gun, and rapid fire breechloading artillery. While both exist before WW1, it was was the first major war where both were employed in large numbers. Those two weapons allowed an army to throw a great deal of artillery shells and bullets at an attacking force that didn't have cover. This high firepower now made defense in depth practical - One or two machine gun positions set up a mile behind the trenches can hold off a very large number of attackers moving on foot across open ground, at a very low cost in manpower to the defenders.

Communications were also very slow, by modern standards. No field radios, and field telephone tech of that day couldn't be rapidly extended, so an attacking force couldn't call down artillery upon the machine gun nests they ran into, once past the trenches. Front line communications were by runner, or on occasion homing pigeons were also used.

The result was stalemate. Too much firepower for a slow moving army to maneuver around the hot spots, or establish a breakout if a trench were overrun. The only real counter to the very high firepower in WW1 was to dig in and get below ground, to limit casualties, and to provide such an obstacle to prevent the other side from attacking.

Even with the offensives that did overrun a substantial amount of trenches, such as the battle of Cambrai, the attackers didn't get very far. The British got about six miles into German held territory, before the Germans regrouped and began bringing down massive artillery on the British forces now out in the open, plus rapidly deployed machine gun positions. That stopped the British army and created a huge number of casualties, so the Germans counterattacked successfully, only to run into the same problem - their forces, now out in the open, were also vulnerable to high firepower defense. The closer the Germans got to the original lines, the more they ran into prepared positions (some made by them). The end result was a minor gain by the British, at a cost of about 90,000 casualties on both sides.

Put simply, the British army couldn't move fast enough to get past not only the trenches, but the secondary defenses behind the trenches, while exposing their troops in the open to artillery fire. WW1 brought unprecedented levels of firepower to the battlefield, before armies had developed tactics to deal with that firepower.

The eventual counter was to move faster than the enemy could react... mobile warfare.

By the outset of WW2, military tech had improved to the point where mobile warfare could be implemented. The German forces simply went around the Maginot line through the Ardennes forest in Belgium and cut off the defenders from supplies and reinforcements.

The key here, as opposed to WW1, was the much higher speed with which the mechanized German forces could move: 20mph versus 1mph, plus being able to communicate with field radios to deal with resistance, either by going around or by calling in their mobile artillery: the Stuka dive bomber.

But, that wasn't an option in WW1. The armies simply couldn't move a large number of troops that quickly or adjust their actions to meet a changing situation with rapid communications, to counter the immense firepower the opposition could bring to bear on them.

The entire front, from the mountains of Switzerland to the north coast of France, was a fortified line (and often multiple lines) of trenches, artillery, barbed wire and soldiers.

Early in the war, Germany managed to push back the anglo-french forces for days. During this time attempts to flank and maneuver where used. Because cavalry was nearly useless against modern infantry (the firepower of modern infantry let it turn, wipe out a cavalry attack, and then continue walking with little harm), the forces trying to chase and flank moved no faster than the forces retreating.

Around about the time that Germany was approaching Paris, the Anglo-French forces turned and attacked. The long chase left weak spots in the German line (and no trenches) resulting in German armies being out-flanked. They retreated and formed a line (still in French territory) and fortified. Once fortified, attempts to attack resulted in ridiculous casualties on the attacking force.

So both sides tried to out flank the other. On one side the Swiss mountains made it impractical -- so they both tried to out flank towards the English Channel. The result was a set of fortifications that stretched right to the ocean (or, in some cases, estuaries of the ocean -- some dykes where breached to form impassible terrain).

So we have a stalemate. Various attempts to break this stalemate and improve the quality of the defences where tried. At Verdun, the Germans tried to use ridiculously intense artillery followed by infiltration tactics to take the high ground in an area they figured the French would want back and where the French supply lines where limited, then proceed to cause so many casualties while defending that the French nation itself would collapse.

As it turned out, the Germans lost almost as many men as the French.

Defence in depth was developed, where the front-line trenches would be held by machine guns and fewer men, and back trenches would hold more men and artillery. When the attackers overran the front trenches (taking lots of casualties) the defending artillery and second line trenches and machine guns would open up slaughtering them, followed by a counter attack against them, with the front-line trenches designed to be undefendable from the "rear".

Both sides developed this over the war.

Tanks where introduced and battle tested. More like tractor-bus hybrids than what we call a tank today. They successfully overran defences, but they initially lacked numbers to create full breach, and by the time the British had enough of them the Germans had started to develop counter tactics.

Finally, after the Russians where knocked out of the war and the USA was threatening to turn the tide, Germany turned its full military might onto the western front. In a massive attack it overran the weaker parts of the defensive line. Stronger areas of the defensive held, and even the flanks of the stronger areas proved unexploitable. Troops in the weaker areas fell back (again, at the same speed the attacking troops advanced) and interior supply lines let the defender form new lines faster than the attackers could advanced. The Germans managed to take useless hellscapes destroyed by previous battles in the war, couldn't maintain supply lines through those hell scapes, got bogged down without defensive works of their own, and fell to a counter attack.

With new supplies and troops arriving from the USA, 100s of Tanks, and tactics developed and refined over the war, the allies proceeded to push back the Germans again and again. German allies started capitulating (often having been defeated in the field), cutting off supplies and opening new flanks.

Despite the fall of the Russian empire, the material, technological (tanks) and supply advantages of the Allies with the addition of the USA had become insurmountable.

Generals tend to "fight the last war."

That said, there are periods of defensive predominance that shape later periods of offensive predominance, and are shaped by earlier periods of offensive predominance. For instance,offensive cavalry ruled supreme between the invention of the stirrup, and the invention of defensive missile weapons such as the long bow and the musket.

Trench warfare, a defensive innovation, made its appearance in the American Civil War (and other late 19th century conflicts) as a result of the offensive Napoleonic campaigns of the early 19th century. Two late 19th century inventions sealed the predominance of trench warfare; barbed wire (to slow ground movement), and machine guns; local small arms fire that was "repeating."

As a result, World War I spawned two inventions that eventually defeated trench warfare; airplanes, and tanks.

The problem was that the World War I airplanes were mostly fighters, not bombers. The role of "bombers" was discovered after the war. The second problem was that while tanks were introduced during World War I, they were then used as mobile infantry, not mobile artillery, because they didn't have the "cavalry" or Blitzkrieg capabilities that made them so effective in World War II. Under those circumstances, successful "side attacks" were infrequent, costly, and historical footnotes.

Both inventions were antidotes to trench warfare. But they weren't fully understood until after World War I, which suffered from the fact that it was fought during a period when defensive warfare enjoyed a "sweet spot."

The existing answers provide detail on why side attacks and real breakthroughs were impossible in practice. I want to add a theoretic level why strategists might also wouldn't want them. To answer your question with emphasis on the "accept" part, I would like to refer you to a military theorist who foresaw some developments and is thus still taught at many military academies today:

Carl Philipp Gottlieb von Clausewitz (*1780; +1831, https://en.wikipedia.org/wiki/Carl_von_Clausewitz)
Sometimes parts of his name are spelled differently: ... Gottfried ... Claußwitz

About 100 years before the first world war, he wrote a famous book "On War", which is also its simplistic title (hence quotation marks). In it he theorized about different strategies and came to interesting conclusions:

• Defenders are at the advantage, because of known terrain and short support lines.
• You should maintain defensive positions and tactics until an attacker has lost enough of his forces, so him being on the defensive wouldn't be enough advantage to him. This point in time is called the culmination point.
• An attacker (or defender on the counter-attack) needs to throw in everything he has and use every means available to avoid getting into the above situation for a counter-attack. Sometimes this is interpreted as favoring "absolute war".
• Using some means cannot be accepted, because war will need to end eventually. You can't use means that will make an enemy refuse to negotiate with you. This is a different interpretation emphasizing "real war can't match absolute war" over the reasons for "absolute war".

Clausewitz applied to trench warfare:

If you do break through an enemy trench, you'll lose your short support lines. You can be cut off and unbalanced. If you win the Race to the Sea and make it around the enemy front, you can still be cut off from your now longer support lines.

So taking from Clausewitz, the answer to your question is: Whoever gives up trench warfare first loses their advantage and may lose the war.

Trench warfare is obsolete

Clausewitz's theories were also applied successfully to tank warfare, in which you would unbalance the enemy by spearheading through the enemy defense lines to disturb the support lines while maintaining relatively short support lines yourself.

He also theorized on asymmetrical warfare.

There were no "sides" where one might perform a side-attack.

After the initial German push was defeated at the First Battle of the Marne, the British and French attacked the Germans at the First Battle of the Aisne. There, both the Germans and the Entente found how effective entrenching was against attacking troops.

Having failed the frontal attacks and having witnessed the advantages that trench warfare gave to the defenders, both sides tried both to attack the enemy at their flanks and to fortify their positions (with more trenches). This was called the Race to the Sea. At the end, the front consisted of an continuous trench system from the North Sea coast to the Swiss border.

So, the sides of each of the pitched battles that happened after that were always covered by trenchs; and the only accepted method to perform an attack on a trench with any possibility of success was to conduct an artillery barrage to wreck the enemy defenses (esp. barbed wire) and defenders before the infantry assault began. This tendency led to a situation where, once a battle had began, it was very lengthy process (due to time needed to transport the artillery) to begin an offensive elsewhere.

sides got locked into relatively short lines of heavily defended trench warfare with little prospect of gains for either side.

The lines on the Western Front were not by any stretch of the imagination "short". The Western Front ran all the way from Switzerland to the Atlantic Ocean.

Side attacks? Well the Race to the Sea (https://en.wikipedia.org/wiki/Race_to_the_Sea) was in fact an attempt by each Army to outflank each other until a continuous front stretched across all of France.

There must have been less heavily defended and dug in points close by, which would have allowed things like cutting supply lines or attacking from the rear.

There were not. The unbroken line of defenses stretching across France was what drove a number of the strategic, operational and tactical challenges that led to the necessary revolutions in military science that ended the conflict, such as the use of tanks and coordinated artillery. It took a lot of trial and error though.

It is pertinent to note in theaters where operational mobility was permitted, both sides attempted to make use of it. This includes Palestine, Prussia and Eastern Europe.

" It seems that the stalemate and immense cost of advancement were accepted as the only way."

Further to note that the scale of technological and industrial sophistication outpaced the military thinking of the time which exacerbated the conditions on the Western Front. Outdated military thinking was a salient feature of the Great War. Consider that up until the stalemate in 1915, the previous wars (such as the Crimean War, The Franco-Prussian War, the Boer War and the American Civil War) also were wars of maneuver.

I would recommend familiarizing yourself with the basic fundamentals of WW1 which are described in some details here: https://en.wikipedia.org/wiki/World_War_I

EDIT: This fantastic article demonstrates how it is easy to judge military thinkers of the day from hindsight.

It is worth noting that, while stalemate and trenches were the experience on the western front, maneuver was the experience on the (less-studied) eastern front.

http://warontherocks.com/2016/07/slaughter-on-the-somme-the-limits-of-foresight-on-the-road-to-the-great-war/