It could be argued that the American Revolution would not have been a success if the Patriots had not received help from European allies—most notably the French. At the time, the Americans had no naval forces that could hope to stand up against the British at sea. As soon as France entered the conflict, it sent its ships into combat against the British navy. This had the effect of diverting British resources and disrupting the resupply of their forces in North America. They also engaged British forces in the Caribbean, Mediterranean, and India, further forcing the British to spread their forces thin.
In 1780, the French helped establish naval bases in the colonies to help the Patriot cause. This began when Rochambeau landed 6,000 French soldiers in Narraganset Bay. These French forces later came under the command of General Washington, who used them to effectively build up his army's strength. The most decisive French contribution to the American victory occurred when de Grasse's fleet defeated the British navy in the Chesapeake Bay on September 5, 1781. This granted control of the bay to Franco-American forces and allowed the French to cut off Cornwallis's possibility of retreat at the Seige of Yorktown leading to the English surrender.
The Netherlands also played its part in aiding the Americans. Even though there was no official alliance between the Dutch and the Patriots, Dutch merchants commenced in trading with them. Dutch weapons and gunpowder made their way to the Continental Army, which was constantly in short supply of these necessary items. The British threatened the Netherlands with military action if they continued to trade with their enemy. When the Dutch rebuffed this threat, the two nations went to war. While there was little actual fighting between the two nations, the British had to divert more of their already thinly spread naval resources to blockade Dutch ports and seize Dutch colonies.
Spain also got involved on the side of the Americans. The Spanish hoped that by supporting the revolution they might be able to regain territory that they had previously lost to the British. Spain's help came largely in financing the American forces through large loans. This money was crucial for outfitting the American army. Spanish intervention in the Bahamas, Florida, and Louisiana also forced the British to divert ships and soldiers away from the fight against the Americans. Like the French, the Spanish navy set about menacing British ships wherever they could find them, further disrupting British trans-Atlantic shipping.
Even Austria was tangentially involved in aiding the Patriots. After the British and French declared war on each other, it was conceivable that the British might attack their enemy on the European continent. However, France's diplomatic alliance with the Austrians convinced British strategists that this would be folly to attempt. As a result, the French were free to attack the British across the Atlantic without worrying about the possibility of a British attack on France itself.