 Doubts over the U.S. president's future are threatening to overshadow the NATO summit that begins today in Washington. Joe Biden is hosting the gathering, but other leaders, including Emmanuel Macron and Olaf Scholz, are also facing their own pressures after recent elections in their countries. Support for Ukraine is a key issue on the agenda, with President Vladimir Zelensky calling for greater resolve from allies in the wake of a barrage of missiles on Ukrainian cities.