In the United States, the conflict between the North and the South became irreversible when?

The first shots were fired by Confederate forces in Fort Sumter, North Carolina. Tensions had been rising, and this was the first act of war that triggered the armed conflict.

Explanation:

A big difference existed between the northern and southerns states throughout the 19th century American growth. The northerners had established a manufacturing industry (and small-scale farming too), but the south based their whole large-scale farming system on the labor of black slaves.

Abolitionist sentiment began to grow in the north and it was important to discuss what was to happen with the new territories of the west. The election of Lincoln (1860) was the final straw (because of his abolitionist posture), and after that South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana and Texas left the Union. After the Fort Sumter battle, Virginia, Arkansas, North Carolina and Tennessee joined the other states and thus began the war.

The conflict between the North and the South parts of the Unites States became irreversible when: Fight erupted between the north and south in april of 1861.

Explanation:

To understand this answer we need to understand the context. First of all, before the fight started in 1861 the north and south of the United States had already experienced very big and critical differences. So, there was already a bad mood between both parties. They had already faced themselves in political, economic, and cultural ways. They were very tense and both looked to answer the latest attack from the other part, even though they were not already in the war. However, when the first fight erupted, it was done. Both parties had too many bad impressions and resentments from the other. Leading to a very bad and devastating war.