Armoured warfare, aircraft, more accurate artillery and the development of reliable motor transport made trench warfare as the world saw it on the Western Front in WW1 obsolete.

The trench stalemate that prevailed in Northern France and Belgium from late 1914 to mid 1918 was brought about by the advancement of weapons technology that strongly favoured the defensive such as the machine gun and quick firing, breach loaded artillery. Combined, these technologies made large scale infantry assaults extremely difficult and highly unlikely to succeed and made the below ground level the safest place to be.

Even when tanks became reliable enough to effectively conduct offensive operations in late 1918, it was more Germany's precarious homefront position and its dwindling manpower that brought it to the negotiating table rather then an outright military defeat.

By the start of the Second World War, weapons technology and motor technology had adavnced to a point where it could be relied upon to successfully carry out offensive operations. Fast moving tanks, aircraft and artillery made static defences such as trenches obsolete for anything more then a temporary shelter from enemy fire. Motorised and mechanised infantry meant that the infantry could move at the same pace as the tanks, allowing a sufficiently organised army to exploit any successful breakthrough by the tanks and to consolidate their positions. Frankly, by the Second World War, the pace of warfare had shifted to a point where no defence could be relied on to do anything other then delay an enemy.

The trench wasn't just a hole in the ground, it was a country spanning machine. The lifeblood of the WWI trench was the rail. With rail it was possible to pour thousands of reserve troops into points along the trench that needed immediate defense, with very short delay.

This machine and its kind of immune system to breaches was able to keep the lines on the Western front fairly static for years but eventually broke down in the face of new tactics, technologies, and simple attrition of basic resources (like rifles, ammunition, men, and food). By WWII this machine was fully obsolete. Armored vehicles supported by wheeled automobiles providing the backbone of the logistics supply chain made blitzkrieg tactics possible and highly effective. Advances in the state of artillery and especially airplanes made fixed fortifications and rail lines much more difficult to maintain.

There are two examples which I think particularly illustrate how much different warfare was in WWII than WWI. One is the Battle of France, where German forces pushed hundreds of thousands of men, thousands of tanks and artillery pieces across a front that was hundreds of kilometers wide. In a matter of days they advanced hundreds of km into the heart of France, bypassing many fixed defenses and causing massive destruction of Allied men and materiel. Another is the Red Ball Express. After the Allied landings at D-Day a logistical supply chain stretched from the US across the Atlantic (on the decks of thousands of Liberty ships) through Britain across the channel to the coast of Normandy (where it was brought ashore on massive floating temporary harbors) and then through France via a convoy of thousands of deuce and a half ton trucks running on regular surface roads operating 24/7 (moving nearly half a million tons of cargo in its roughly 3 months of operation).

What these examples show is the extreme mobility and flexibility that automobiles (including armored vehicles) provides. In this environment fixed defensive lines can often be a detriment because they just serve as a convenient location for enemies to attack with artillery, bombers, etc.