Shared enemies builds closeness by uniting two or more people against a common adversary. It runs on social identity dynamics (Tajfel and Turner): defining an out-group instantly sharpens in-group belonging, and few things fuse a group faster than a threat to unite against. The resulting solidarity is real and fast, which is why it appears everywhere from healthy coalitions and unions to office cliques, factional politics, cults, and radicalization pipelines.
The technique becomes manipulative when the enemy is the primary substance of the bond rather than an incidental fact of it. Legitimate alliances are held together by shared positive goals and can tolerate members having ordinary relationships across the line; engineered ones are held together mainly by opposition, escalate the enemy’s menace over time, and treat contact with the adversary as betrayal. That trajectory — expanding out-group, narrowing circle, disappearing nuance — is the pathway by which grievance bonds harden into extremism.
Because the feeling of righteous solidarity is genuinely satisfying, this bond is hard to question from inside. The defensive test is diagnostic rather than moral: ask what would remain if the enemy disappeared. A relationship with its own positive substance survives that question; one manufactured from shared hatred collapses, which tells you what was actually holding it together.