We show that in the Sherrington-Kirkpatrick model at inverse temperature $β$ with uniform external field $h>0$, replica symmetry holds in the regime $ β^2\mathrm{E}[ \mathrm{sech}^4(β\sqrt{q}Z+h)] \le 1$, where $Z$ is a standard Gaussian random variable. This confirms a prediction of de Almeida and Thouless (1978). The proof proceeds by a direct analysis of the Parisi measure using the characterization provided by Jagannath and Tobasco (2017).