The Second Continental Congress did some very important things to create the United States and to get it organized. It also was important for the fact that it led the colonies (or the country, depending on how you want to look at it) through the Revolutionary War.
The Second Continental Congress convened soon after the Battles of Lexington and Concord started the fighting in the Revolutionary War. At that time, it was still not clear if the colonies were going to fight for independence or just for some sort of change in the way they were governed.
This uncertainty showed in one of the most famous things the Congress did. This was the "Olive Branch Petition" of July, 1775 in which the Congress tried to patch up the colonies' differences with England. When that failed, the Congress took its most important and famous action. This was when it issued the Declaration of Independence in July of 1776.
The second most important thing the Congress did was to create the Articles of Confederation. This was the first constitution of the United States and it served as a way to keep the colonies/states together through the war and the first few years of independence.
The most important actions the Congress took then, were the Declaration of Independence and the Articles of Confederation. It also ran the country as much as was possible during the Revolutionary War.
When the First Continental Congress adjourned, they agreed to meet again nine months later if necessary. Since a lot of things had occurred since the First Continental Congress adjourned, they did meet in May 1775. There were many actions that they took.
After the Battle of Lexington-Concord, the Second Continental Congress created an army under the leadership of George Washington. Many people believed that it was only a matter of time before we would go to war with Great Britain.
The Second Continental Congress also began to print money. The money would be used to pay for supplies for the military. A committee to deal with other countries was also formed.
The Second Continental Congress also tried to communicate with the King that the colonists wanted peace. The Second Continental Congress sent the Olive Branch Petition to the King. It stated that we wanted to remain peaceful with Great Britain. However, we wanted certain policies changed. The King rejected the petition.
The Second Continental Congress also authorized the writing of the Declaration of Independence. Once it was written, the Second Continental Congress eventually adopted it. Once that was done, we had declared our independence from Great Britain.