From VOA Learning English, welcome to The Making of a Nation, our weekly program of American history for people learning English. I'm Steve Ember.

In 1823, President James Monroe introduced one of the most important foreign policy decisions in American history. It became known as the Monroe Doctrine.

The doctrine said the United States never had, and never would, take part in any war between the European powers. At the same time, it warned the Europeans against interfering in the western hemisphere. Monroe declared that the Americas are "not to be considered as subjects for future colonization by any European powers."

Historian Harlow Giles Unger says the Monroe Doctrine marked the end of the colonial era.

"The United States now considered the entire western hemisphere our sphere of influence. That we would keep out of their affairs but they must keep out of our affairs."

The United States continued to grow. New states joined the Union. Indiana, Mississippi, Illinois and Alabama all became states before 1820.

Louisiana had earlier become the first state to be formed from part of the Louisiana Territory that the United States bought from France. The rest of this huge area was called the Missouri Territory. By 1819, there were enough people in part of the Missouri Territory for that part to become the state of Missouri.

But becoming a state required the approval of Congress. And historian Harlow Giles Unger says admitting Missouri would have changed the balance of power in the Senate.

"The Senate gave each state two votes. And by convention since the signing of the Constitution, the Senate was evenly divided between slave states and non-slave states. So the admission of Missouri would have added a slave state to the Senate and left the northern non-slave states as a minority. And they were not going to accept this."

Slaves were not new in America. Spain had brought them to the West Indies hundreds of years before. In 1619, a ship brought 20 African slaves to Jamestown, Virginia. These black men were sold to farmers.

Over the years, the use of slaves spread to all the American colonies.

However, there were many more slaves in the agricultural South than in the North. The farms in the North were smaller and needed less labor. But in the South, farms were much larger. Slaves were the least expensive form of labor.

Most of the northern states had passed laws before 1800 freeing slaves. Even the southern states made it illegal to import more slaves from Africa. But those southerners who already owned slaves believed they were necessary, and they refused to free them.

Slavery had been legal when France and Spain controlled the Louisiana Territory. The United States did nothing to change this when it purchased the territory. So slavery was permitted in the Missouri Territory at the time Missouri asked for statehood.

A New York congressman, James Tallmadge, offered an amendment to Missouri's request to become a state. Tallmadge proposed that no more slaves be brought into Missouri, and that the children of slaves already there be freed at the age of 25. His proposal started a debate that lasted a year.

Supporters of Tallmadge argued that his proposed amendment was constitutional. The Constitution, they said, gave Congress the right to admit new states into the Union. This also meant, they said, that Congress could refuse to admit new states unless these states met conditions demanded by Congress.

Supporters of the amendment also said small farmers of the North and East could not compete with the southern farmers and the free labor of slaves. They argued that these northern and eastern farmers had as much right to the land of Missouri as anyone else. The Louisiana Territory had been paid for by the taxes of all Americans.

Those opposed to slavery also argued that slave-holding states would be given too great a voice in the government if Missouri joined them.

Under the Constitution, only three out of every five slaves were counted in the national population. The census taken every 10 years is used to set the number of members for each state in the House of Representatives. In the House. unlike the Senate, the number of votes that a state has is based on its population.

In the past, each time a slave state was admitted to the Union, a free state had also been admitted.

Harlow Giles Unger explains what the supporters of the amendment may have been thinking:

"The problem basically was not so much a moral problem. From their point of view, it was as much moral as economic. Because the northern states could not compete with southern states. Northern states paid their labor by the piece. In the south slave labor was free of charge. So the south had a tremendous economic advantage. They could produce goods at much lower cost than the north. And the advent of a majority in the Senate would have tilted the balance of power."

Southerners had an answer for each argument of those supporting the Tallmadge amendment. They agreed that Congress had the constitutional right to admit or reject a state. But they said Congress did not have the right to make conditions for a territory to become a state.

William Pinkney of Maryland argued that states already in the Union had joined without any conditions. If Congress, he declared, had the right to set conditions for new states, then these new states would not be equal to the old ones. The United States would no longer be a union of equal states.

The debate was intense on both sides.

The House of Representatives passed the Missouri bill with the Tallmadge amendment. But the Senate rejected it.

The people of Missouri would try again for statehood when the new Congress met in 1820.

By this time, another free state was ready to enter the Union. Maine, with the permission of Massachusetts, asked to become a separate state. The Senate joined the Maine bill with the one for unconditional statehood for Missouri. Senators refused to separate the two, and so they continued to debate about conditions for statehood and slavery.

Finally, Senator Jesse Thomas of Illinois offered a compromise. He said Maine could be admitted as a free state and Missouri as a state permitting slavery. But he said no other state allowing slavery could be formed from the northern part of the Louisiana Territory.

Many southerners were not satisfied. The compromise closed the door against slavery entering large new areas of land. Southerners, like any other Americans, had a right to settle in the new territory.

The Senate accepted Thomas' compromise. Congress approved statehood for both Missouri and Maine. Now, President Monroe just needed to sign the bills.

It was the spring of 1820. James Monroe was coming to the end of his first four years as president. He wanted to be elected again. But he faced a difficult decision about whether to allow the Missouri Compromise.

President Monroe owned slaves. He understood the feelings of the South. His friends urged him to veto the compromise bill, because it limited slavery in the territory. He also understood the strong feelings of those who opposed slavery.

Monroe believed the compromise was wrong — but not because it kept slaves out of the territory. The president did not believe the Constitution gave Congress the right to make such conditions.

Monroe even wrote a veto message explaining why he could not approve the compromise. But, in the end, he did not use his veto.

He believed there might be civil war if he rejected the compromise. So Monroe signed the bill. Missouri had permission to enter the Union as a slave state.

The crisis seemed to end. But a few months later, a new problem developed. Missouri wrote a state constitution that it sent to Congress for approval. One part of this constitution did not permit free black men to enter the state. A number of lawmakers in Congress immediately opposed the state constitution. They said it violated the United States Constitution.

The United States Constitution said citizens of each state had the same rights as citizens of each of the other states. And since free black men were citizens of some states, they should have the right to be citizens of Missouri. The debate lasted several months.

Former House speaker Henry Clay finally proposed a compromise that both sides accepted. Missouri could become a state if its legislature would make this promise: it would never pass any law that would violate the rights of any citizen of another state. This second compromise ended the dispute over slavery in Missouri and the Louisiana Territory.

The compromise actions of 1820 settled the crisis of slavery for more than 20 years. But everyone knew that the settlement was only temporary.

Former president Thomas Jefferson expressed his feelings with these words: "This momentous question, like a fire bell in the night, awakened and filled me with terror. I considered it at once as the knell of the Union. it is hushed indeed for the moment.

"But this is a reprieve only," he said, "not a final sentence."

The effect of Monroe's decision and the election of 1820 will be our story next week.

I'm Steve Ember, inviting you to join us next time for The Making of a Nation – our program of American History from VOA Learning English.