What major political developments took place during Thomas Jefferson's administration?

The most important political development which took place during the Jefferson administration was the Louisiana Purchase.  The United States grew considerably after this purchase from Napoleon Bonaparte, and many in the United States were in favor of this expansion. Jefferson also sent a combined force of naval ships and Marines to defeat the Barbary Pirates—he considered this to be more cost-effective than constantly paying bribes.   

Jefferson's foreign policy was not altogether favorable, however.  Jefferson...

The most important political development which took place during the Jefferson administration was the Louisiana Purchase.  The United States grew considerably after this purchase from Napoleon Bonaparte, and many in the United States were in favor of this expansion. Jefferson also sent a combined force of naval ships and Marines to defeat the Barbary Pirates—he considered this to be more cost-effective than constantly paying bribes.   

Jefferson's foreign policy was not altogether favorable, however.  Jefferson cut the size of the army and navy, and this would be detrimental during the War of 1812.  Jefferson feared that a strong military could be used to curtail the people's freedom.  To protest Britain and France stopping American cargo on the high seas, Jefferson passed the Embargo Act which prohibited trade with the two warring nations.  Unfortunately, the two nations were the United States' greatest trade partners, and the economy soon crashed. The losses were felt most acutely in New England.  Jefferson eventually lifted the Embargo Act, but New England, never a friend to Jefferson, was now more against the Virginian than ever before.