I think the goal, for the Germans, was to keep the US neutral for as long as possible. In the event that it didn't happen they wanted Mexico on their side since Canada would obviously be with Britain and the US. I think the idea was to have a front on the North American continent, making it harder for the US to send her troops to Europe in large numbers. They promised Mexico generous financial aide and I think they offered to help Mexico take back Texas in the event of the US losing the war. But Mexico saw through it and realized it was pretty much blowing smoke and didn't make sense for their country.