After the Roman Empire split, Constantine the Great moved the capital to Constantinople and reunified Rome, which was later divided into the Western and Eastern Roman empires. The Eastern Roman Empire (Byzantine Empire) is called the second Rome, and its capital is Constantinople (now Istanbul).
In A.D. 1453, the Sultan of the Ottoman Empire led an army to capture Constantinople, and the last Byzantine emperor Constantine Xi died in battle, and the Byzantine Empire perished. Later, the Russian Grand Duchy also claimed to be the successor of the Byzantine Empire. Later, Ivan IV, the Russian Grand Duke, was crowned czar, renamed czar Russia and became the third Rome. Capital: Moscow (later moved to St. Petersburg).
In fact, the Holy Roman Empire has nothing to do with the Roman Empire. The full name of the Holy Roman Empire is: the Holy Roman Empire of the German nation, which is called the First German Empire by later generations.