Sometimes it is very difficult for non-native speakers to understand the meaning of 'would'. So, why do these sentences use 'would'? If I change 'would serve' to 'served', is this wrong? What is the difference betweeen 'served' and 'would serve'? If I change 'would reach' to 'reached' or 'would become' to 'became', do these sentences then have different meanings?
In all, more than 4,000,000 Americans served in the armed forces during the first World War. And 16 million would serve during the second one.
(the first one is 'served', why is the second one 'would serve'?)
And, historians say, the sauce was probably first created in a Chinese community in northern Vietnam. Later, this sauce would reach Indonesia and be called kecap.
(why not 'reached'?)
William Henry Harrison would become the ninth U.S. president.
(why not 'became'?)
Why do these example sentences need the word 'would'?