A lot of the problems in both WWI and WWII was because many heads of the State underestimated the scale of the war and went into battle without a good preparation such as resources or weaponry in battle or generals who had an antiquate idea of how to fight, but the times the soldiers were left at their own devices they proved to be quite capable, even Rommel was impressed by the soldiers skills, often quoted by saying "The German soldier has impressed the world, however the Italian Bersagliere soldier has impressed the German soldier", also interestingly is the Raid of Alexandria, with 6 divers who got captured, but they managed in their escape (and subsequent capture) to disable two battleships>https://en.wikipedia.org/wiki/Raid_on_Alexandria_%281941%29>In the end, all the divers were made prisoners, though they succeeded in severely damaging both Queen Elizabeth and Valiant, disabling them for nine months and six months respectively. Eight members of Queen Elizabeth's crew were killed. Sagona lost her stern section and the destroyer HMS Jervis, one of four alongside her refuelling, was badly damaged. Neither of the two capital ships sank but they were out of action for several months.
Also in WWII there was in Russia a problem that the soldiers (who were equipped for an Alpin fight) got sent with little to no preparation side by side with the germans in their position, but their equipments wasn't made to withstand the winter conditions.
TL;DR: good soldiers, shitty generals