a) In these sentences, Roosevelt argues that the United States cannot stay isolated and be only concerned with what goes on within its borders. Roosevelt compares this to China's behaviour, and he argues that such a behaviour will only lead the country to more problems. Instead, the United States needs to prepare to face conflict strongly.
b) Roosevelt argues that this change is necessary because the problems that the country has will not go away on their own. If they are dealt with swiftly, the nation might have a chance of improving the situation. However, if they are not dealt with bravely, then they will never be solved.