Why were trenches good defences/difficult to attack?

Question

So far, I have:
- It took a lot of skill to hit land about 1.5 metres across
- France & Belgium were incredibly flat and so you could see a soldier breaking the skyline for miles and you could shoot them.
- The sandbags absorbed shrapnel and protected the trench from caving in [after rain].
- Barbed wire was difficult to get through.

Any others?
Thanks!

Answer 1

You have missed the obvious. Two words - the invention of the machine gun. The Heavy Machine Gun invention meant that 2 men (or however many people it took to operate one) lying in a trench could kill a whole attacking army with a standard firepower of 500 rounds a minute with a 4,000 yard range. Good defense, poor offense. But you wouldn't get far trying to attack with machine guns as they were heavy, and you would be fully exposed attacking while the enemy was fully protected by their trench.

As for the barbed wire. They were also designed in a funnel formation. So that it channels the attacking army to slowly group together in one final point where the machine gunners could concentrate shot in one basic area.

Answer 2

The very nature of tactical warfare had changed and nobody had noticed. Prior to the American Civil War most infantry rifles were unrifled muskets, with limited range, accuracy and killing power. During the civil war futile frontal attacks caused massive causalities which were unheard of during earlier periods. Trenches were also used extensively during the ten month siege of Petersburg. In fact, almost 25% of the Union Army's existence was spent in siege operations.
Technology had not made sufficient advances to overcome the inherent power of the defensive in World War 1 till late in the war. Tanks, aircraft, innovative infantry tactics, larger and more accurate artillary all contributed to the demise of static defenses.
Of course after World War 1 few noticed this change until after the fall of France.

Answer 3

Unless a shell exploded directly above the trench you were protected. Also you were hidden from the enemy's snipers.
Trenches were always zig zagged and never in a straight line. This was so that if the enemy got into your trench they couldn't just fire along the trench and hit everyone in it.

Answer 4

attackers are completely exposed if they try to rush the line.