Trench warfare wasn’t overcome in WWI; the Germans were beaten primarily because they ran out of fresh bodies and the ones still alive refused to continue fighting. The much-Canadian-ballyhooed “rolling bombardment” certainly helped a little here and there, but even the famed Canadian attack at Vimy Ridge took only a few miles of ground and had no significant impact on the German line as a whole, for the same reason all the other attacks failed; without radios, the exploitation of attacks was not possible. Furthermore, artillery of the sort available in 1914-1918 simply wasn’t, and isn’t, effective at killing troops in entrenched positions.
Trench warfare still works in many circumstances. Its utter domination of the Western Front from 1914 to 1918 was a unique circumstance where the dominant weapon, the machine gun, just happened to have no effective counter.
However, there are a large combination of developments that make it possible to break heavy trench lines:
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Tanks. Machine guns no longer can be used to close an entire battlefield.
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Volume of bombs. The artillery of 1914-1918 was primarily dependent upon fragmentation shells, which were most of the available shells and which are utterly worthless at attacking trenches. High explosive was not available in sufficient quantity and even monster artillery bombardments like the preparatory bombardment of the Somme were not really firing enough HE at German positions to do any serious damage to their trenches and dugouts. If you actually add up the HE fired per square kilometre, it’s shockingly low - anyone who added it up should have realized it wasn’t going to work.
However, by WWII, the ability to drop bombs with vastly more accuracy AND the vastly greater availability of high explosive made it possible to drop a horrifying amount of ordnance on a position for the purpose of breakthrough. See Operation Cobra.
- Radios. Many, many times in WWI, attackers DID drive past the trench line. But without the ability to coordinate followup attacks they stalled and were soon confronted with new defensive lines.
Radio also VASTLY magnifies the effectiveness of artillery - I mean, makes it 10, 20, maybe a hundred times more powerful. People get hung up on shell size and volume, but the artillery of 1945 was nothing like the artillery of 1918. There is no comparison. Artillery bombardments in 1914-1918 were predetermined, wide-ranging attacks that preceded attacks and couldn’t concentrate on targets of opportunity. By 1945 the radio control of artillery enabled an army to throw a huge volume of artillery on a newly identified target, or in support of a breakthrough, and that’s on top of having guns with more HE, rocket artillery, better fuzes, and all that stuff. U.S. Army artillery was absolutely and without question the most effective and underrated weapon of WWII; it could throw so much so fast at a new target that a rumour started in the German army that the Allies had invented an automatic howitzer.
Whatever else is true about war, remember this; artillery is the god of war. Has been for a long time.
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Airplanes. Able to attack fixed positions, interdict defensive troops regrouping behind the lines, etc. Consider the difference between German defenders in 1916 and 1944. In 1916 the Germans would have time to move reserves into position behind a threatened position. In 1944, any German vehicle moving in daylight was quite likely to be attacked by an Allied or Russian bomber. Trains were routinely blown right off the tracks. It was a constant struggle for them to fill holes in defensive lines.
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Trucks. It’s not just tanks that allow an army to move around; wheeled vehicles allow for much greater freedom of movement.
Trenches have been a dominant force in warfare since 1854 and will be for a long time to come. They played critical roles in WWII on all fronts. They just happened to be really important in 1916, just as aircraft carriers were really important in 1942 or Minie-ball rifles were really important in 1863.