Using both the analytic steepest-descent method and computer-simulation techniques, we investigate the classical easy-magnetization-plane antiferromagnetic chain in a magnetic field. The free energy as well as the static correlation functions are determined analytically by means of a mapping onto the soliton-bearing double anisotropic ferromagnet. The magnetization and the specific heat are calculated numerically, and the results are compared with the analytical findings and with experimental measurements on the linear-chain antiferromagnet (CH3)4NMnCl3 (TMMC). We discuss the soliton contribution to the thermodynamics, static correlation functions, and the switching of hard-magnetization-axis phenomena. Furthermore, the importance of quantum effects in TMMC is considered.