How did trade and warfare make this
civilization strong?

The trade and warfare make civilization stronger because they promote the economy of civilization.

Explanation:

Whenever there is civilization there happens the production to meet their daily survival needs. later there exist a surplus good which makes them to export and to promote the trade.When there is a trade-in civilization so there exists a new way of improvement in civilization which also indirectly promotes the economy of civilization.  To safeguard the wealth and prosperity, we have to defend the enemy to defend them warfare is required. Thus it also preserves our wealth and makes our civilization stronger.

President Kennedy did not like the spread of communism in Vietnam,

Explanation: When President Kennedy and the whole White House heard bout this news, they took action. They declared war on North Vietnam, by sending troops to Vietnam soil, sending money and weapons to fight Vietnam, Kennedy did not want the world to follow the steps of Communism, so that is why he acted so instantly and furiously against Vietnam!