Imagine an America without the Texas annexation and the Mexican War. Would we ever have acquired the west? I suppose Mexico may have eventually offered to sell it to us.
I read somewhere that the Gadsden purchase could have been MUCH larger, including Baja and several Northern Mexican states, but one side or the other eventually balked.
As I recall the sale price for Gadsden was enormous-—over $60 billion in modern money. Santa Anna needed cash to rebuild his army. There were no polls then. Some historians claim Mexicans would have welcomed a part in the US, but most think the Mexicans were still very hostile (I agree). No one but Santa Anna could have sold Gadsden. The Mexican legislature already rejected a railroad land rights deal across the isthmus that he approved. Some say had the South had a chance to obtain more land in the Mexican Cession, but remember that there was intense opposition to adding more slave territory to the US. We briefly, in 1854, had a chance to get Cuba but by then the South was obsessed with Kansas.