He helped the United States win the war and secure (or get) its freedom.
When the war ended in 1783, Washington was tired of fighting.
He wanted to return to his home in Virginia and work on his plantation (or big farm).
However, once he was there, he began to understand that there were many problems with the Articles of Confederation (or the legal document that existed before the Constitution).
He decided to help the other Founding Fathers create a new legal document for the country and went to participate in the Constitutional Convention.
The other members of the Constitutional Convention unanimously voted (or all voted in agreement) to make him the president of the Convention.
The Constitution was written under his leadership in 1787.
Just two years later, in 1789, George Washington became the first president of the United States.
George Washington had a major (or very important) influence (or effect) on the United States as the country was developing.
That is why he is often called the "Father of our Country."