Why is this quote by Julius Caesar so ironic?

Question

Act II, Scene 2

lines 10-12

Caesar shall forth. The things that threatened me

Ne'er looked but on my back. When they shall see

The face of Caesar, they are vanished.

Answer 1

He's saying that the things that threatening approach him from the back, or that he turns his back on them.  When he turns and faces them head-on, they flee like cowards.  

Caesar was stabbed IN THE BACK, caught by surprise, by his friends.  That's why it's ironic.

Answer 2

I guess because he got stabbed in the back?  He's saying that when he faces his threats he makes them vanish or takes care of them.   Well....except for that one time huh?