The Haldane conjecture, when applied to the 2D O(3) Heisenberg model with classical spins, predicts a mass gap at theta=0 and a massless phase at theta=pi. We have simulated the model at imaginary theta and extrapolated the results to real theta in order to verify this conjecture. Our result is that the model becomes critical at theta=3.10(5) in good agreement with the expectation.