This could be Asimov's "Homo Sol" (ISFDB entry showing it's been in a lot of collections).
Tan Porus, master psychologist at Arcturus University, is invited to join the delegation to be sent to administer the invitation, but sends one of his assistants. The delegation returns in despair; not only have the Solarians refused the invitation, but they exhibit irrational and war-like behavior that contravenes the sacrosanct psychological laws established over millennia by Federation psychologists.
In opposition to his doubting colleagues, Porus insists that the hominids of this new planet are something special: a species susceptible to panic en masse. "What of Kraut's Law," a colleague asks him, "which says it is impossible to panic more than five hominids at a time?" Porus dismisses this with scorn, manipulates the Federation Council into sending him to Earth to demonstrate his thesis, and in a short time broadcasts back the results of his demonstration: "Panic, morons! World-wide panic!" Porus is eventually able to work out the psychological means to convince the Solarians to accept membership in the Federation.
As per the description here:
when humans invent FTL and send a
spaceship to Alpha Centauri to settle the 5th planet, our
civilization is invited to join the galactic federation. The
advanced races are fascinated that we've outdone their technology,
with an improvement of "hyperatomic" drive. Humans modify various
peaceful alien technologies into deadly weapons.