When first reading Othello, I found it unbelievable to be honest. Why would Othello be taken in so easily by Iago? But you need to put it in the proper context because without it, it doesn't make any sense. Othello is insecure as a Black man living in Venice and married to a White woman. He's self conscious of that, despite the fact that he is highly regarded in the military. He trusts Iago as one of his closest friends and believes he has his best interests at heart, though Iago does not as he's terribly jealous of Othello. This happens every day, even in modern contexts; that's what the show Jerry Springer is all about. I also highly recommend watching a stage or movie production of this play as it makes it much more believable.
I say all of this because the initial major events in and of themselves don't seem that major unless you take those factors into consideration.
1. Desdemona drops the handkerchief she is given by Othello accidentally. It was an important present between the two of them. Iago has Emilia steal it and he tells Iago that she gave it to Cassio.
2. Iago has Cassio talk about his lover, Bianca without mentioning her name in front of Othello. Iago continues to goad Othello, convincing him more and more that Desdemona is having an affair with Cassio (a White man from Venice).
3. Othello begins abusing Desdemona in front of her family. She is bewildered by this and denies the affair until Othello smothers her to death in a jealous rage.
4. When Othello finds that Desdemona never lied to him, he commits suicide.