The Frontier Thesis was the idea that American Democracy was innately tied to the American Frontier, and that the frontier needed to continue expanding for democracy to flourish. Settlers on the frontier were able to experiment with new ideas on how best to establish towns and local government. Historically, the western frontier was traditionally more liberal than conservative, older states back east. This propelled the advancement and consistent innovation of American Democracy. Frederick Jackson Turner presented the idea in 1893 at a conference in Chicago. At the time, the U.S. land frontier to the Pacific had largely faded as California was admitted to the union in 1850. The Philippines became a foreign U.S. possession in 1898 and many politicians thereafter insisted on its admittance to the union, including Theodore Roosevelt. 

Manifest Destiny was the belief that American settlers were destined to conquer the North American mainland through the exceptionalism of the American people and its unique democratic institutions. In Gast's painting, Lady Liberty is marching along with settlers from the bright and enlightened east to the dark and unknown west. Behind her, trains forge west while in front of her, "savage" Indians are running further into the darkness, away from the settlers. Further indications of the time frame include the covered wagons making the journey west and the farming techniques that were well-implemented in the east in the foreground of the painting. 

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Constantly at war with his European neighbors, Napoleon's coffers emptied quickly. In order to replenish his financial stores, Napoleon looked to sell land in the Americas, finding a very willing buyer in Thomas Jefferson. With the Louisiana Purchase, Napoleon was able to soldier on while the United States added a huge swath of new territory.

In 1867, Secretary of State William Seward purchased Alaska from Russia for a paltry 7.2 million dollars. Although some criticized the purchase at the time, referring to it "Seward's Folly," reaction to the move was generally positive. The move had the effect of weakening both English and Russia colonies in the region, and the natural resources found in Alaska have benefited the country greatly.

Oregon Country referred to the territory that encompasses nearly all of the Pacific Northwest, including British Columbia, Idaho, Oregon, Washington, and parts of Wyoming and Montana. Those in favor of the slogan hoped to gain all of the region for the United States. The border was set, however, at the 49th parallel.

The Battle of Fort Sumter was fought between Union and Confederate forces from April 12-14, 1861. After this battle both sides rallied tens of thousands more troops, and the American Civil War began in earnest.

The Three-Fifths compromise was a compromise reached by delegates at the 1787 Constitutional Convention regarding how slaves were to be counted for census purposes, so that would not be the best answer here. The Louisiana Purchase was the agreement that purchased a large swath of territory from France, so that would not be a good choice. The Mason Dixon Line was a surveying project in the early 19th century to demarcate the boundaries of Delaware, Pennsylvania, and Maryland, so that would not be the best choice. The Articles of Confederation outlined the first, short-lived government of the U.S., and so that would also not be the best choice. The Missouri Compromise was put forward by Henry Clay, and it was a federal statute that regulated the practice of slavery in the new western territories, so that would be the best choice.

The Civil War forced an innovation in naval warfare as both the Monitor and the newly renamed Virginia were newly ironclad. This was a large departure from previously purely wooden ships. Even though both ships were able to ship other non-ironclad ships during the battle, they themselves sustained little to no damage throughout the engagement and the naval battle was inconclusive. However, neither ship fought ever again after the battle. 

On April 9, 1865, General Robert E. Lee's Army of Northern Virginia officially surrendered to General Ulysses S. Grant's Union Army at Appomattox Court House in Virginia. Although there were other Confederate armies throughout the south, Lee's Army of Northern Virginia was the largest and best-trained force in the confederacy. Their surrender effectively ended all violence in the war and led to the subsequent surrender of all confederate forces thereafter. 

President Lincoln made several assurances that his administration would not initiate civil war. All aggressions early in the war were perpetrated by Confederate forces, finally culminating in the declaration of war after the attacks on Fort Sumter.