Why would an army want to flank its opponent, or attack its opponent from the side, instead of head on?

Head on is where the enemy is bet prepared possibly. It's where their weapons are already facing possibly and it's where they best see any attacks. From the side, they might not see it as much and would also not have their weapons their, like how a person holds their fists in front of them and not at their sides typically, I suppose. If they go from the back, however, the enemy won't be able to see them much, like if someone snuck up behind you, and their backs might not be guarded as well as their front, like how, once more, if a person has their fists in front of them and expects a person to come from there but they come behind them, they can attack with the other seeing or knowing possibly and get a few hits in before the other turns around, tries to flee, or tries to attack back.

the answer is a new goverment.