Final answer:
Andrew Carnegie and Thomas Scott built and donated a national telegraph system to the Union armies during the war, improving military communications.
Step-by-step explanation:
During the war, Andrew Carnegie and Thomas Scott played a crucial role in military communications by building a national telegraph system. They funded the construction of the system with their own money and then donated it to the Union armies. This allowed for quicker and more efficient communication between military headquarters and the battlefield, enabling better coordination and strategic decision-making.
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