He sees the Old World as the cause of the destruction of his home. The Enclave invented the package that was sent by the NCR, who based many of their government ideals from America. The package then speaks to the nuclear bombs, invented by America that detonates the Divide. He saw the symbol of America and when the Courier delivered the package, he saw how one man could change the course of history forever.

Really, America was a super Empire that reached out to the Divide even after death and caused all of the evil that happened within that place. Ulysses hates the Old World, as seen by him vilifying the NCR and see them as children following values they don't even understand. He also hates House, saying he should be silenced and not bring the Old World back.

Ulysses wants a New World. He respects you more if you stand beside values that aren't related to the Old World: Caesar's Legion establishing a new, more hivemind oriented Empire or the Courier shaping an independent Mojave that's unrelated to the Old World.

Yo, i respect that guy, and i dont think he would have wanted to bomb the Legion (and NCR but he has to prove he′s point somehow) but i am eleminating threats of a independent nation trying to survive in a post nuclear apocalypse (50 style :/ ), not svcking ONE guys dike...

Was just reading the wiki entry and when he saw the divide it was filled with old world symbols (American flags, and military symbols) he wants to reawaken America becauseit contained places like the divide, very strong and wealthy yet peaceful, not like Caesar or NCR who although are strong, aren't very peaceful. He sees them as having no future but afer it's destroyed he learns of the nukes there from the think tank and wants to destroy NCR allowing Caesar to take over because he believes in Caesar more, probably due to the fact that he fought for the legion and NCR was the one who actually sent the detonator.