Final answer:
Peter the Great gained territory along the Baltic Sea through a long war with Sweden, leading to Russia's emergence as a major Northern European power.
Step-by-step explanation:
Peter the Great acquired territory for Russia along the Baltic Sea through his efforts during a prolonged conflict, established as a historically significant war. It was not the Ottomans, nor a treaty with Qing China, nor a war with Denmark that allowed Russia this territorial expansion. Rather, Peter the Great of Russia gained substantial Baltic territories as a result of a long war with Sweden, known as the Great Northern War (1700-1721), which resulted in Russian success and the establishment of Saint Petersburg in the newly acquired region.
In pursuing his goals, Peter the Great embarked on a grand tour of Europe to learn about shipbuilding and administrative techniques, which he later used to modernize Russia and extend its influence. This ambition came to fruition as he fought Sweden and secured access to the Baltic Sea, thereby making Russia a formidable Northern European power, which was instrumental in Russia's evolution into a vast and powerful empire by the 19th century.