You could be... You can declare it and apply for a visa, which you might be refused, or you can omit it, go under the usual visa waiver scheme, and hope Tony Blair hasn't told Washington about it. If it was a long time ago you're probably safe. If recent, then hard to predict.
Questions very similar to this come up at least once a week on AB.
The 'official' answer is that you won't be able to travel to the US because you require a visa which takes at least 4 months to obtain.
The 'unofficial' answer is that the US authorities probably won't know about your conviction because only information on certain types of offence (e.g. involving drugs, explosives or potential terrorist links) is passed on from the UK. So, as long as your assault conviction doesn't relate to an incident arising from a protest outside the American embassy, you should proably be OK.