The general rule is that, during a declared war, all intercourse, correspondence, and traffic between U.S. citizens and subjects of enemy states that might be advantageous or provide comfort to the enemy are prohibited. For example, it is illegal to transmit money across enemy lines. In addition, a U.S. citizen cannot lawfully make a contract with a citizen of an enemy state while war exists, and any such contract is, therefore, void. The laws of war proscribe all trading with the enemy and all other commercial relations while a state of war exists.
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