Why Was Caesar's Action of Crossing the Rubicon a Declaration

Why was Caesar's action of crossing the Rubicon a declaration of war?

A) A Roman commander intending peace would have dissolved his army and command prior to entering Italy.
B) Caesar had already declared himself dictator of Rome.
C) Caesar had promised his soldiers bonuses if they followed all of his orders.
D) Caesar had not yet completely defeated the Gauls.
E) Only Pompey was permitted to take such an action legally.