Rome made a treaty with Israel.  They would not invade if Israel came under Roman rule of law and kept the peace.  Israel agreed with one stipulation: they would not bow to Caesar or acknowledge him as god.  That condition was agreed upon and war was averted.  The Israel nation could continue to worship their God.

The Pharisees were put in charge of law and order and keeping the peace in Israel--with one exception: they could not condemn anyone to death.

And Jesus threatened their position.  The people followed Jesus--and the religious rulers saw Jesus as a danger to their power agreement with Rome.  So...they plotted from the beginning to get rid of him.  Problem solved.  Things would continue in Israel with the Pharisees at the helm.  Rome would stay out of their business.  The people would once again look to the religious leaders instead of this upstart called Jesus.

But when Jesus arrived in Jerusalem on a colt, with hoards and throngs of people laying down palms and praising him as their King, the religious order realized that they had to convince Rome to act--Rome reserved the power to eradicate someone.  Thus, the plan to take Jesus when the crowd had gone home, bring him before the Roman magistrate and end the problem of who was going to remain in charge of the Jewish people--the Pharisees, or this man Jesus that the crowd called their King. They made a plan to nab him and convinced the Roman authorities to crucify him. 

But He wouldn’t go away, he would’t stay dead.   His power grew and the world was changed forever.  He rose from the grave and lives.  And is coming soon.  Soon.  Get ready.  Rome won’t be in charge next time.