For alternate history, historical, and whatif conjecture!

In the 1840s the US was beginning to eye up Mexican lands in the west such as California and Texas after its rebellion against Mexico. Mexico at the time was struggling since many of their states had little love for the central government.

When the war kicked off the Mexican army was outclassed by the American army and under generals like Zachary Tailor and Winfield Scott they were able to defeat Mexico under Santa Anna, this would also help make the way for future civil war leaders such as Robert E Lee and Grant.

But what if the war never happened say Mexico gained a European ally like France or Britain and was slightly more together politically to have more states be willing to come and led troops to the Army.

Now how would this effect America, would they still gain the west coast and how would no war effect Mexico and also if so how would this effect the American civil war if it still happens