Putin, while I'll admit that I haven't read much of Mearsheimer, my source is "International Relations Theories: Disciplines and Diversity" Chapter 4, starting on page 85 (I don't really expect you to look it up, but I just wanted to let you know I'm not bullshitting my reference).
@Fasces - you are underestimating the capabilities of Iran. Our military doesn't worry so much about Russia anymore because Russia doesn't threaten us. Iran has significant Anit-Acces, Area Denial weapons systems. In 1995 (I think) the U.S. Navy held a drill pitting a carrier group against an asymmetric military force of hundreds of small attack boats such as Iran has. In that drill, "Iran" won. The Persian Gulf is too shallow for our subs to operate in effectively. Iran just took down one of our drones. For more info, read "Why AirSea Battle" by Andrew Krepinevich, published by the Center for Strategic and Budgetary Assessments
Oh, and the British Empire question is an interesting one (just read an article on this). You see, after World War 1, the western powers took on a defensive realist stance. That is, in WW1, Germany sustained so many casualties and was unable to advance for so long that they assumed that in military technologies defense had the advantage. They thus felt secure staying behind their fortifications. Defensive realism, going along the theories of Kenneth Waltz, maintains that nations judge threats on 4 areas: Aggregate Power (which the West was equal to Germany in), Proximity, Offensive Capability (which they believed Germany did not have), and Offensive Intentions (which they underestimated in Hitler). Thus balancing of power is encouraged and aggression is discouraged. Indeed if France had stationed a tank division in the Ardennes in 1940, things might have been very different on the western front (not saying France wouldn't have fallen, but Germany would have gotten a much bloodier nose).