Final answer:
Peter I, or Peter the Great, was the absolute monarch who sought to modernize Russia and secure warm water ports through territorial expansion.
Step-by-step explanation:
The absolute monarch who toured Western Europe for ideas on modernizing his nation and who invaded other countries to secure warm water ports was Peter I, also known as Peter the Great. During his reign, from 1672 to 1725, Peter I embarked on a grand tour of Europe in disguise to learn about shipbuilding and administrative techniques, which he then applied to modernize Russia and establish the Russian Empire. He was particularly interested in expanding Russia's territorial holdings to include warm water ports that would be accessible year-round, leading to conflicts with neighboring states such as Sweden, from which he took the region including St. Petersburg.