expression, godan verb (-ru)
plot an assassination
A set phrase meaning to secretly plan the murder of a prominent person. The verb 謀る (はかる) here conveys scheming or conspiring.
彼らは大統領の暗殺を謀った。
They plotted the assassination of the president.
Compound of 暗殺 (assassination) + を (object marker) + 謀る (to plot). The verb 謀る is a native Japanese word with the core meaning of scheming or planning something secretly, often with negative connotations.