to compel;  to force;  to oblige
The situation compelled us to take up arms in self-defence; Under the force of circumstances, we had to take up arms in self-defence;  Circumstances forced us to take up arms in self-defence
I was compelled to stay; it was compulsory for me to stay
compulsory;  mandatory;  obligatory
Is English a compulsory subject? -  No, it is an optional subject
Attendance at birthday parties is not mandatory
It is obligatory to show respect for one's teachers