One of the main reasons why Congress did not initially annex Texas is because it would create an imbalance of free and slave states in Congress.
During the early to mid 19th century, the United States was constantly expanding their territory. This was due in large part to the concept of manifest destiny. As the US was spreading their influence, they ran into the problem of whether or not these new territories and states would have the institution of slavery.
Southern states favored new territories and states having slavery while Northern states did not favor slavery in these new territories. The reason behind the Northern states includeds:
1) Influences from the abolitionist movement.
2) Northerners did not want slave states to have more representatives in Congress than they did.
This caused Texas annexation to be delayed, since Texas would be admitted as a slave state. This would throw off the balance of free and slave states represented in Congress, giving slave states the advantage.
1 is the correct answer
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You might look up John Green's "Crash Course" episode on the Industrial Revolution. In his treatment of the subject, Green gives a quick rundown of reasons why the Industrial Revolution started in Britain and Europe, which he notes are essentially ethnocentric reasons.
1. It is sometimes argued that the Europeans had a "superior rationality" (stemming from the Enlightenment). That's really just a cultural superiority argument, claiming Europeans are better and smarter than others.
2. Only Europe, it is said, had a strong culture of science and invention to make such inventive, innovative creations possible. It's true that there was such a strong inventive atmosphere -- but there have been similar cultures of science and invention in various times in history at various places around the globe
3. The freer political institutions (especially in Britain) encouraged innovation, and personal property rights in their political systems created incentives for inventors. There's some truth to that, but great inventiveness hasn't always happened in democratic countries. The Soviet Union made it into space before the United States did, for example.
4. It is said that the small population in European nations required labor saving inventions, because less labor was available. But the opposite argument could also be made. The larger a nation's population (someplace like China), the more they would want and need industrial advancement to make society work better for everyone.
Ethnocentric arguments tend to be made by persons within the ethic group favoring such arguments. It's better for us to take a global view and consider all perspectives.