On Mexico's national flag, the eagle perched on the cactus represents what the nomad tribe of the Mexica people (who would become the Aztec tribe) believed was a sign from their god to indicate where to build their new city. The snake was a misinterpretation from the Spanish, and the concept stuck.
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It marked the end of the hunter gatherer ethos and the beginning of what we call civilized societies.
For the most part, they wanted an opportunity to own their own land. European land was largely owned by rich Lords and Monarchs. The US was largely open for the taking, and the poor would agree to be an indentured servant for many years in exchange for a chance to make it to the US, and hopefully one day own their own land.
Nominating conventions related to politics because at nominating conventions political parties seek to decide who they will nominate to run in different elections. Nomination conventions are often times the place where the parties formally announce the nomination of a candidate who has been chosen by a caucus or electoral process either in a state or nationally.
Colonists who lived in the region where the proclamation line did not touch, were angered because they were forced to move towards the cities and ocean. They felt that choosing a home was more important than being under a monarchy that would move them away.