Is liberty defined more by politics or economics? While America remains the world's sole superpower, numerous rising great powers re-grade the global landscape, making it flatter than it's been at any time since the early 20th century. Some experts naturally expect we'll face all the same strategic challenges of that age, including great power war, while others are more sanguine, judging this era's dynamics to be quite different. Two new books by political commentator Robert Kagan and journalist Fareed Zakaria provide a nice contrast along these lines.