No George Washington was not the only president to fight so its false other presidents also served in the military
Answer: Not sure exactly when were talking about but overall no, racisms was so bad at points that I wouldn't be surprised if you were shot by your own side simply because of the color of your skin.
Answer:
A. Because the 13th amendment was passed to abolish slavery, white business owners and plantation owners used the Black Codes as a way for the white employers to still treats blacks as slaves rather than employees.