In recent years there has been an increased interest in laser pulses in the mid-infrared and terahertz regimes in the strong-field community. Here we present the recently developed adiabatic theory for molecules [PRA 101, 053422 (2020)]. This theory exploits the slowness of the nuclei and the variation of the external laser field compared to the electronic motion, and is thus well suited to describe interactions with long-wavelength pulses. It treats nuclear motion in addition to all interactions among nuclei, electrons and the laser field. We will demonstrate the accuracy of the theory by comparing with exact solutions of the time-dependent Schrödinger equation. We will also show an application of the theory to explain why the energy width of the vibrational state distribution in the molecular ion shrinks when isotopes with heavier nuclear masses are used. *This work was supported by funding from the JSPS.