The ANSWER is:
George Washington believed that the success of the nation rested in the Union, but recognized that both internal and external conflicts could disrupt it.
Explanation:
If all members of the union stay united, the union as a whole will have more strength, be safer and foreign nations will not be tempted to challenge the union or its tranquility. At the same time, it will help avoid conflicts among the members. That´s Washington´s message.