suru verb (irregular), transitive verb
to set a boundary; to bound
Rare literary transitive suru verb meaning to establish or mark a boundary between areas. In modern Japanese, 境を接する or 境界を定める are far more common.
その川が二つの国を境している。
The river forms the boundary between the two countries.
Derived from the noun 境 (さかい, 'boundary') combined with the verbalizing suffix する. The exact historical derivation is uncertain; the form is rare and largely supplanted by periphrastic expressions.