US President Joe Biden will meet 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. Questions surrounding Biden’s mental fitness for office, however, loom over the summit, as the US presidential race heats up between the president and his predecessor, long-time NATO critic Donald Trump. Outgoing NATO Secretary-General Jens Stoltenberg said last week that leaders will also discuss his proposed plan to provide Ukraine with military aid worth €40 billion ($43 billion…