Why two competitors can erase all profit

In the Bertrand model, just two firms selling an identical product and competing purely on price drive that price all the way down to marginal cost, wiping out economic profit — the same result as under many-firm perfect competition. French mathematician Joseph Bertrand proposed it in 1883 as a rebuttal to Cournot's earlier model, where firms compete on quantity and retain pricing power.

— Joseph Bertrand, Book review of Cournot's Recherches — Journal des Savants, 1883