I. phrasal
or set upon
: to attack with violence : assault
soldiers were set upon by thugs loafing around barracks … who knocked down their victims and rifled wallets — Dixon Wecter
II. transitive verb
1. obsolete : to set on foot : promote
2. : to urge (as a dog) to attack or pursue : unleash
3. : to incite to action : instigate
set on to rebellion by their leaders
4. : to set to work : set about
an extra gang was set on to the job
intransitive verb
: to go on : advance