So, what would you have us do?
In 1950, North Korea attacked South Korea. The US and other nations came to South Korea's defense. General MacArthur came up with a daring landing behind enemy lines in North Korea, and soon North Korean troops were retreating. But MacArthur kept pushing northwards, further and further into North Korea and closer to China. China issued a warning not to cross a certain line, and when MacArthur went further, China sent hundreds of thousands of troops southwards. Only by incredibly brave fighting did the US and its allies avoid total disaster. After a couple more years, the Korean War ended in a temporary cease-fire that has lasted over 60 years.
China will protect North Korea. An unprovoked attack by us will lead to a major military conflict.
So, what should those presidents have done?