Like many young Europeans of his time, he was enthralled with the Revolutionary activity in the New World. Shortly after arriving in Philadelphia in 1776, Kosciuszko read the Declaration of Independence and he recognized everything in which he truly believed. When he discovered Thomas Jefferson was responsible for drafting the Declaration, he had to meet him. A few months later, while moving south with the Continental Army, Kosciuszko stopped in Virginia to meet with Jefferson. The two men spent the day comparing philosophies and became the best of friends.
The colonists were desperate for engineers, even those who had only just arrived from abroad with no knowledge of America, and on October 18th 1776 Kosciuszko became Colonel of Engineers, Washington's chief engineer and strategist. The thirty-year-old began planning forts along the Delaware. His first duty was to help fortify Philadelphia from naval attack. Kosciuszko centered the defenses on a new fort, Mercer, while setting up aquatic blockades designed to force British ships closer to both the shore and bombardment. Kosciuszko moved on to help with the defense of Fort Ticonderoga. Partly due to disregard of Kosciuszko's advice, Ticonderoga was toppled. Kosciuszko's forces felled pine trees and flooded fields to slow the pursuit of the British. This gave the rebels time to prepare for their first major victory of the war: Saratoga. At Saratoga, Kosciuszko fortified Bemis Heights overlooking the Hudson. His ingenious design contributed to the surrender of 6,000 troops under General John Burgoyne.
Kosciuszko then undertook the defense of the Hudson at West Point in 1778. So thorough were his fortifications, that the British never mounted an assault. One of the more imaginative links in the colonel's defensive plan was a 60 ton chain stretched across the Hudson to block British ships. Kosciuszko went on to lead troops and, by 1783, he had been promoted to Brigadier General. Of the foreign subjects who came to the revolution's aid, Kosciuszko's contribution was perhaps only second to Lafayette (with Charles Lee and von Steuben far behind). Kosciuszko learned how to win battles with a militia of untrained and poorly equipped men, as well as how to apply his years of study, often quite brilliantly, in the field, abilities he would use in his later career.
Kosciuszko developed a strong dislike for slavery and serfdom based on his belief in individual rights and Republican government. These applied not just to the newly independent colonies, but to his homeland.
Following the Revolutionary War, Kosciuszko returned to Poland to fight for independence from the occupying Russians. In 1789, he became Major General of the Polish forces. The reforms of the May Constitution of Poland, the first modern constitution in Europe and second in the world after the American, were seen by the surrounding powers as a threat to their influence over Poland. In, 1792 a Russian army of 100,000 crossed the Polish border and headed for Warsaw, now that Russia and her imperial allies were no longer battling the Ottoman Empire; thus began the War in Defence of the Constitution. the Polish Army was well-trained and prepared for the war. After the betrayal of Prussian allies, the Army of Lithuania could not stop the advancing Russians. The Polish Army was too weak to oppose the enemy advancing in the Ukraine and withdrew, regrouped, counter-attacked, and was victorious. In the ensuing battles, Kosciuszko repelled the numerically superior enemy and became the most brilliant Polish military commander of his time. In 1792, King Stanislaw joined the Targowica confederation and surrendered to the Russians and, in 1793, Prussia and Russia signed the Second Partition of Poland. Such an outcome was a blow for the Targowica Confederation who saw their actions as defence of centuries-old privileges of the upper classes, now regarded by the majority of Polish population as traitors. After the partition, the Targowica confederation evaporated.
Kosciuszko prepared a plan of an uprising and led the Polish-Lithuanian uprising of 1794 (known as the Kosciuszko Rebellion), winning key conflicts before being defeated by the vastly superior forces of Prussia, Russia and Austria.

Kosciuszko at Raclawice by Jan Matejko. Krakow National Museum. Events of April 4, 1794. Kosciuszko, on horseback, reviews his formations. He is greeted enthusiastically by peasants, armed with pikes made from reshaped scythes, who contributed significantly by their valor to a stunning victory in a battle against superior Russian forces during the initial phases of the Kosciuszko Uprising.Kosciuszko drew popular support from peasant classes as well as nobles and magnates. Wounded during the battle of Maciejowice, Kosciuszko was taken prisoner. After two years' incarceration, the Czar granted him amnesty on the condition he never return to Poland.