Paul I of Russia
Paul remained overshadowed by his mother for most of his life. His reign lasted four years, ending with his assassination by conspirators. He adopted the laws of succession to the Russian throne—rules that lasted until the end of the Romanov dynasty and of the Russian Empire. He also intervened in the French Revolutionary Wars and, toward the end of his reign, added Kartli and Kakheti in Eastern Georgia into the empire which was confirmed by his son and successor Alexander I.
Nicholas I of Russia
With two older brothers, it initially seemed unlikely Nicholas would ever become Tsar. However, as Alexander and Constantine both failed to produce sons, Nicholas remained likely to rule one day. In 1825, when Alexander I died suddenly of typhus, Nicholas was caught between swearing allegiance to Constantine and accepting the throne for himself. The interregnum lasted until Constantine, who was in Warsaw at that time, confirmed his refusal. Additionally, on 25 (13 Old Style) December, Nicholas issued the manifesto proclaiming his accession to the throne. That manifesto retroactively named 1 December (19 November Old Style), the date of Alexander I's death, as the beginning of his reign. During this confusion, a plot was hatched by some members of the military to overthrow Nicholas and to seize power. This led to the Decembrist Revolt on 26 (14 Old Style) December 1825, an uprising Nicholas was successful in quickly suppressing.