A.
After the Civil War Abraham Lincoln was just the first of many US Presidents to encourage immigration through economic and politic measures (one of them was to give free land to anyone willing to work it). The country needed people to rebuild itself and to settle the west of the continent. And everyone was aware that there is plenty of space.
<span>The Union army occupied the South to ensure that African-American freedmen enjoyed their full civil and political rights.</span>
The greatest advantage of United States in world war II was its ability to rapidly transition from peace to war and mass produce weapons and war equipment at a colossal scale. This was so effective that the USA was able to make up for time lost, and the nation was able to effectively train the necessary forces and then exert a massive material superiority.
America was able to build up an air force that came to dominate the skies, and with this air superiority, it was all but over for the enemy, the Axis forces.
They migrate due to altitudes during different times of the season
