This week, NATO leaders will gather in Washington, D.C., for the defensive alliance’s 75th annual summit. The high-profile gathering comes at a moment of political turmoil in the United States, as President Joe Biden struggles to demonstrate his fitness to lead following a poor debate performance late last month. The 81-year-old Biden, by his own admission, had a very “bad night” as he faced off against former President Donald Trump on the debate stage. But the perception of an enfeebled U.S...