For the past few months, the world has been paying a lot more attention to the war in Iran than the war in Ukraine. But the two conflicts have something in common. They both involve nuclear-armed military superpowers that have found themselves unexpectedly stymied by much smaller countries. That’s a new reality of warfare. Today I’m writing about why Vladimir Putin is feeling newly under pressure, why Volodymyr Zelensky is feeling emboldened — and why big powers with the urge to launch a war of choice might want to think…