I'm picturing the Greek Phalanx formation, where soldiers were arranged in rows and the men in Row B or even Row C could attack the enemy by thrusting their spears between the soldiers in Row A.
Can you do that with a Reach weapon in DnD 5e? Attack an enemy even though there's an ally (or even a row of allies) directly between you and them?
Yes, but they will have half cover
The rules on cover are listed here. The total cover rule is the only one that prevents attacking:
Total Cover
A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.
Allies don't give total cover though, they give half cover:
Half Cover
A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.
So you can attack "through" an ally but the target will have half cover from the attack.
