Between the world wars
eisenhower's presidency
the 1960s
what is the organizing principle of this list?

The organizing principle of that list is CHRONOLOGICAL ORDER.

"Between the World Wars" would cover the years from November 1918 (end of World War I) to September, 1939 (start of World War II in Europe).

Eisenhower's presidency came after World War II.  Eisenhower was in office as President from January, 1953, to January, 1961.

"The 1960s" would be the next chronological item in the list, starting at the end of the Eisenhower years and covering the next decade.

b) the confederacy had to pay all union war expenses.