The South became more dependent on slavery, deepening the differences between the South and the North
The US innovation of Afghanistan was a response to the 9/11/01 attacks on the US. So A.
Because the soviets were with the nazis at that time and europe was going to be taken over during world war II
Think about the idea here and you'll see how the idea of "cost" is inevitable in every decision. (It's true not just of governments, but of our own decisions too -- but we'll focus on governments here.)
Let's say the government decides it wants all citizens to have access to health care. Well, that's going to cost dollars to pay for that health care. Where will those dollars come from?
Let's say the government decides, in response to school shootings or other acts of gun violence, to ban certain types of guns or ammunition. That costs something to the gun dealers who were making money off those sales (and they'll object). Or let's say the government decides to do further and deeper background checks on all gun buyers. Well, that will cost something in terms of personnel and processes to accomplish all the background checks. Or let's say the government decides to increase mental health screenings and treatment because persons with mental illness issues may become violent and dangerous to society. That will cost much in order to organize and carry out better mental health intervention across the country.
I focused on just a couple issues there (health care, gun control). But the same principle holds on anything government does. You can think about your own examples that you'd want to use. Anything the government decides to do comes with some sort of costs attached. That doesn't mean it's bad to make such decisions -- it just means we need to count the cost and invest our efforts where they will have the best benefit.
