The Mississippi River has historically been extremely important for the growth of the United States. Since it connects such large parts of the US, it was used for trade and ships would go through it all the time with goods which was great for the economy. Goods could also come to New Orleans from Europe and be taken north all the way as up as Minnesota.
The two oceans were very beneficial in various ways to the United States. For example, the large bodies of water could be used as natural defense against foreign incursions since in early colony days it was not possible to sustain an army that would have to attack the US. They were also great source of resources such as food. Whaling was historically particularly important for the development of the regions around the oceans.
The great plains have historically been really important for the economy because it was the primary area in which cattle ranching was developed. Because of the vast fields cattle could be grown there easily and in large numbers and this are provided food for most of the United States while also benefiting the economy from successful business endeavors.
Abundant natural resources make it possible for the United States to manufacture goods and to also trade with other nations. Since the US has so many various things, from food to minerals to mines to pretty much anything, they are self-sufficient to themselves and don't have to rely on trading partners if they don't want to.
At his first press conference as president, Reagan audaciously questioned the ... the largest in American peacetime history, was undoubtedly another factor
