  We discuss the construction of the two-flavour axion-pion effective
Lagrangian at the next-to-leading order (NLO) in chiral perturbation theory and
present, as a phenomenological application, the calculation of the decay rate
of a GeV-scale axion-like particle via the channel $a \to \pi\pi\pi$. Through
the NLO calculation, we assess the range of validity of the effective field
theory and show that the chiral expansion breaks down just above the kinematic
threshold. Alternative non-perturbative approaches are called for in order to
extend the chiral description of axion-pion interactions.
