Tusk claims the veto undermines Poland's sovereignty.
The EU loan amounts to €43.7 billion.
Tusk's stance could lead to a significant political conflict.
Briefing: Poland’s PM Tusk defies president’s veto over €43.7 billion EU defence loan
Strategic angle: A power struggle erupted in Poland as Premier Donald Tusk insisted on accessing EU defence funds despite President Nawrocki’s veto.