The Cunning of Iago in Othello

So Iago is this extremely interesting, cunning, evil character. But over all of these trait’s Iago’s jealousy is what drives him to scheme and plot to take down othello and weave his elaborate web of destruction. Does anyone know exactly when in the play does Iago’s intense jealousy come into play? In the very 1st act scene one iago and roderigo are arguing in the streets and iago starts to rant about his hatred for Othello and how Othello passed him up over cassio for promotion to ancient.
Iago had an enormous amount of battle experience, experience that Othello has witnessed firsthand “And I, of whom his eyes had seen the proof At Rhodes, at Cyprus, and on other grounds Christian and heathen, must be belee’d and calmed” while cassio has no battle experience and only knows the theory of leading men into battle, “I have already chose my officer. And what was he? Forsooth, a great arithmetician,One Michael Cassio, a Florentine(A fellow almost damned in a fair wife)That never set a squadron in the field,Nor the division of a battle knows More than a spinster—unless the bookish theoric, Wherein the toged consuls can propose As masterly as he. Mere prattle without practice is all his soldiership.
But he, sir, had th’ election he has no clue how to put into practice this theory and is therefore unqualified according to iago. Later on in the play iago shows his jealousy of Othello because Othello is rumored to have slept with his wife toIago becomes jealous of cassio getting the promotion over him so he devises a plot to get revenge.