How did westward expansion exacerbate tensions over the institution of slavery?

The balance between the powers of the slave states and free states was heavily, well, imbalanced.

Explanation:

During the expansion to the west, many different people brought their ideological views, as well as proofs of their ideological views to the west in hopes of finding a new lifestyle as well as richness to a certain extent in the "golden hills of California". However, with these clashes of ideas came the explosive fights of ideas that bore arms. Tensions increased as the South saw the expansion towards the west as a race against the ever growing strength of the North, who had a larger population and already stronger influence on the government, and the North saw the expansion towards the west as creating safe havens against the spread of slavery, as well as an expansion towards their outlook of the Free-Soilers.

The balance of power was thin, and many different solutions were put forward to hold on to the uneasy peace, the 36°30' treaty was the most significant, in which south of the latitude, slavery was allowed, while north of the latitude, slavery was disallowed. California, being split into two sections, was allowed to be a free state in the effect that the Northern states would help catch fugitive slaves on the run to Canada, in exchange. This made many Northerner's extremely unhappy, while the South was extremely unhappy on the loss of prospect of land that could, in turn help inflate their power in Congress. This was one of the many sparks that would soon end up for what we know as the Civil War.

the allies' desire to seek retribution for germany's actions