The stars in the Milky Way and its satellites contain detailed information about galaxy formation and evolution; indeed, stars record the past in their ages, photospheric compositions, and kinematics.
To specifically understand the details of stellar evolution, planet formation and galaxy formation and evolution, a uniquely powerful approach is to measure the chemical abundances in photospheres of large numbers of stars over the full age range, with and without known planets, as here, buried in stars, lies the historical record of past events throughout the large volume in which these stars live.
A comprehensive survey of chemical abundances in statistically robust samples including the most ancient stars, the rarest of objects, will allow us to unlock this information back to the earliest times.
The Milky Way remains a crucial testbed for exploring all the related physical processes in detail. The different groups of chemical elements play important roles in understanding the variety of events that have occurred over time in Galactic history, given that they are produced by different types of stars, with different timescales. For instance, the alpha-elements track back the early star formation episodes, being produced by supernovae Type II in short timescales. The ratio with iron-peak elements, produced on longer timescale, is a well-established tracer of the star formation efficiency and the shape of the initial mass function (IMF). Carbon, nitrogen and oxygen (CNO), as well as neutron-capture elements, are essential to constrain the physical conditions and mixing in stellar interiors, and also to trace the sta formation across cosmic times as they depend, through the mix of nucleosynthetic channels, on the star formation history, interstellar medium matter cycle and the mass function; they can also track mass-accretion events that disrupt the timeline, and thus ages. Neutron-capture elements have recently also been shown to be useful chemical clocks, although an open debate is still ongoing about their nucleosynthesis channels.
WST will permit large, statistically robust samples of precise measurements of a wide range of chemical elements, as well as radial velocities  (kinematics) in stars in the Milky Way and in our local galactic neighbours. Disentangling complex events happening over billions of years requires knowing the ages, masses and chemical composition over a wide range of elements and 3D motions of the stars. 
Gaia has clearly shown the power of this approach, providing parallaxes and proper motions to complement radial velocities and precise chemical composition from large spectroscopic surveys of local stars. Considering both the high- and medium-resolution modes, WST will target more than a few tens of million stars, probing a larger volume in the Milky Way, and a wider range of nearby galaxies, than any other current or planned facility.
Noticeably, WST will be a treasure trove for the study of the conditions that drive planet formation and evolution, by being able to explore in a consistent and detailed way all the exoplanet host stars (50-100k stars) to be discovered over the Southern sky by missions like PLATO, Gaia and the Rubin and Roman telescopes. The large number of exoplanet hosts discovered by these missions cannot all be followed up in high resolution.