US President Joe Biden meets his fellow NATO leaders on Tuesday for a landmark 75th anniversary summit aimed at projecting strength and shoring up support for Ukraine in its defence against the Russian invasion. However, questions surrounding Biden’s mental fitness for office loom over the summit, as the US presidential race heats up between the president and his predecessor, long-time NATO critic Donald Trump. Biden has resisted calls to withdraw from the presidential race and may use the summit to dispel doubts over his health, while NATO tries to entrench its support for Ukraine in the even…