A deal to buy California and settle the Texas boundary line--Slidell worked as an ambassador to Mexico to offer $25 million for California and agreement to set the boundary line of Texas at the Rio Grande.
Slidell brought this deal to Mexico partly because Texas was a free independent state that wanted to be annexed by the US but had not settled their southern border. The US wanted the boundary settled before annexing Texas. Also the wanted the California region to expand land holdings. Mexico rejected the offer.