1. Most of the United States has a temperate climate, but Hawaii and Florida are tropical, Alaska is polar/arctic, the Great Plains region is semiarid (dry, almost desert-like), and the Great Basin of the southwest is an arid (desert-like) climate

2. The U.S. can be divided into many different regions (areas that share some common characteristics).  By clicking on the map to the left, you can see the United States divided into five geographic regions: The Western Mountains & Basins, the Great Plains, the Central Lowlands, the Appalachian Mountains, and the Atlantic Coastal Plain. This is only one way to classify the very diverse areas of America.

3. Barriers to Expansion
One of the greatest obstacles for early settlers intent upon moving westward, were the Appalachian Mountains.  They extend, in an almost unbroken chain, from Maine to Alabama, with very few places to pass through.  Early settlers quickly found the Cumberland Gap (at the junction of modern-day Kentucky, Tennessee and Virginia) to be one of the easiest ways westward (and inland) from the colonies.
Four hundred miles west of the Cumberland Gap, settlers would encounter the largest river in North America,  At that point, the 2,552 mile long Mississippi River was nearly one mile wide, and over 100 feet deep.  Travelers had to go miles upstream to find a ford (a place to cross the river).
Once across the Mississippi River, the Great Plains made for fairly easy travel.  It was not until settlers saw the Rocky Mountains, that westward expansion slowed.  Most people were diverted either far to the south, or chanced high passes through the mountains, sometimes getting stranded in unpredictable winter snowstorms. Of course, early Spanish settlers had been venturing inland from the sixteenth century, and had made many contacts with the indigenous peoples of the southwest.

4. General Impact and Colonization Patterns
U.S. history is full of examples where geography directly influenced the development of the nation.  For example, at the end of the French & Indian Wars, the Proclamation Line of 1763 was established.  It forbade the (then) British colonists to pass beyond a designated western borderline which roughly followed the Appalachian Mountains.
This angered colonists even further because they had to pay for British soldiers to patrol the area.  Many colonist ignored the decree, moving west and settling anyway.  This led to increased resentment of white settlers on the part of Native Americans, and serious problems in the future..

5. In 1803, Thomas Jefferson oversaw the
purchase of the Louisiana Territory from France.
For only 15,000,000 America doubled its geographic area. Unfortunately, it was difficult for settlers to get through the Appalachian Mountains, and migrate westward. By 1825, The Cumberland Gap (at the conjunction of Kentucky, Virginia and Tennessee) and the Erie Canal  were funneling the majority of migrants westward.  By this time, residents of the newly independent United States felt no compulsion to obey by treaties signed between the Indians and the British.  Tensions were growing.
In 1848, gold was discovered in California.  But, in order to get there, travelers had to sail around South America, or ride through the desert southwest.  In 1874, gold was found in the Black Hills of South Dakota.  The rush continued.

6. Territorial Expansion
The Homestead Act of 1862 gave federally owned land to anyone who settled it.  Any adult (aged 21 and older) could claim a 160-acre lot, by farming it for only five years, and own it after paying a fee at the end of that time.
It was during the rush westward that Americans realized the need for a transcontinental railroad connecting California to the rest of the United States.  The new railroad through the Great Plains made it possible for farmers to ship their goods for sale in the East, but it crossed lands considered inviolate by the resettled Indians.  Open warfare broke out in several locations in across Montana, Kansas and the Dakota territories. White settlers demanded protection from the so-called savage Indians, which in turn led to the killing of thousands of Native Americans.

7. By 1871, Native Americans had been made wards of the state by the federal government.  They could no longer make individual treaties with the federal government, and most lived on federal reservations.  The Dawes Act of 1887 tried to Americanize Indians by abolishing all tribes, and giving former members 160-acre farms (on the reservation) that they would own outright after 25 years. 
The Dawes Act , and others like it, were supposed to provide a road to American citizenship, but more often destroyed native cultures.  It was not until 1924 that all Native Americans were made legal citizens of the U.S. Clearly, natives were treated much differently than whites. 

8. Wartime Not surprisingly, it is often in the midst of a war, or shortly afterward, that territorial expansion  occurs.  During the French & Indian War, British territory expanded toward the Appalachian Mountains.  The Louisiana Purchase was made because Napoleon needed money to finance his expansionist ambitions.  Florida was gained between 1810 and 1819, years surrounding the War of 1812.

9. In 1846, war against Mexico (over the Texas Annexation in 1845) yielded Texas, California and the rest of the Mexican Cession to the U.S.  America and Great Britain agreed to terms over the Oregon Country in 1846.  Much of the Great Plains was settled during the Civil War, and Alaska was purchased from Russia in 1867.
In February 1898, the American battleship Maine suffered an explosion while in the harbor in Havana, Cuba.  American newspapers whipped up a fury, and in April 1898, war was declared against Spain.  The U.S. won the war in just a few months, and got Puerto Rico, Guam and the Philippines out of it.  In 1903, the U.S. signed a treaty leasing the Panama Canal Zone.