Prince Karl Eitel Friedrich Zephyrinus Ludwig of Hohenzollern-Sigmaringen, was born in Sigmaringen. He was the second son of Karl Anton, Prince of Hohenzollern-Sigmaringen and his wife, Princess Josephine of Baden. After finishing his elementary studies, Karl entered the Cadet School in Münster. In 1857 he was attending the courses of the Artillery School in Berlin. Up to 1866 (when he accepted the crown of Romania) he was a German officer. He took part in the Second War of Schleswig, particularly at the assault of the Fredericia citadel and Dybbøl, experience which would be very useful to him later on in the Russian-Turkish war.
Although he was quite frail and not very tall, prince Karl was reported to be the perfect soldier, healthy, disciplined, and also a very good politician with liberal ideas. He was familiar with several European languages. His family being closely related to the Bonaparte family (one of his grandmothers was a Beauharnais and the other a Murat), they enjoyed very good relations with Napoleon III of France. Romania was, at the time, under the influence of French culture and Napoleon's recommendation of Prince Karl of Hohenzollern-Sigmaringen weighed heavy in the eyes of Romanian politicians of the time, as did his blood relation to the ruling Prussian family. Ion Br?tianu was the Romanian politician sent to negotiate with Karl and his family the possibility of installing Karl on the Romanian throne.
The former Romanian ruler, Alexander John Cuza, had been banished from the country and Romania was in chaos. Since his double election had been the only reason the two Romanian countries (Wallachia and the Principality of Moldavia) were allowed to unite by the European powers of the time, the country was in danger of dissolving.
Young Karl had to travel incognito on the railroad Düsseldorf-Bonn-Freiburg-Zürich-Vienna-Budapest, due to the conflict between his country and the Austrian Empire. He travelled under the name of Karl Hettingen. As he stepped on Romanian soil, Br?tianu bowed before him and asked him to join him in the carriage (at that time, Romania didn't have a railroad system).
On May 10, 1866, Karl entered Bucharest. The news of his arrival had been transmitted through telegraph and he was welcomed by a huge crowd eager to see its new ruler. As a proverbial sign, on the same day it had rained for the first time in a long period of time. He pledged his oath in French: "I swear to guard the laws of Romania, to maintain its rights and the integrity of its territory".
Immediately after arriving in the country, the Romanian parliament adopted, on 29 June 1866, the first Constitution of Romania, one of the most advanced constitutions of its time. This constitution allowed the development and modernization of the Romanian state. In a daring move, the Constitution chose to ignore the country's current dependence on the Ottoman Empire, which paved the way for Independence.
Article 82 said "The ruler's powers are hereditary, starting directly from His Majesty, prince Carol I of Hohenzollern-Sigmaringen, on male line through the right of first-born, with the exclusion of women and their issue. His Majesty's descendants will be raised in the Eastern Orthodox Religion."
When he was elected prince of Romania, Carol was not married and, according to the Romanian Constitution he himself had approved, he was not allowed to marry a woman of Romanian origin. In 1869, the prince started a trip around Europe and mainly Germany, to find a bride. During this trip he met and married princess Elizabeth of Wied. Their marriage was one of the most unfitted matches in history, with Carol being a cold and calculated man while Elizabeth was a notorious dreamer. They had one child, Princess Maria, born in 1871, who died on the 24th of March 1874. This led to the further estrangement of the royal couple, Elizabeth never completely recovering from the trauma of losing her only child.
After the proclamation of the Independence (1877), Romania was effectively a kingdom. From 1878, Carol held the title of Royal Highness. On 15 March 1881, the Constitution was modified to state, among other things, that from then on the head of state would be called king, while the heir would be called royal prince. The same year he was crowned King.
After the proclamation of the Kingdom (1881), the succession was a very important matter of state. Since Carol's brother, Leopold, and his oldest son, William, declined their rights, the second son of Leopold, Ferdinand, was named prince of Romania and heir to the throne. Elizabeth tried to influence the young prince into marrying her favorite lady in waiting. According to the Romanian constitution, though, the heir was forbidden from marrying a Romanian. As a result of the affair, Elizabeth was exiled for two years, till after Ferdinand's marriage to Princess Marie of Edinburgh.
Towards the end of their lives, though, Carol and Elizabeth finally found a way to understand each other and were reported to have become good friends.
In 48 years of rule (the longest rule any Romanian principality has ever known), he helped Romania gain its independence, he raised its prestige, he helped redress its economy and he established a dynasty. In the Carpathian mountains, he built Pelec Castle, still one of Romania's most visited touristic attractions. The castle was built in German style, as a reminder of the king's origin. After the Russo-Turkish war, Romania gained Dobruja and Carol ordered the first bridge over the Danube, linking the new acquired province to the rest of the country.
The long rule of Carol helped the quick development of the Romanian state. But, towards the end of his reign and the start of the World War I, the German-born king wanted to enter the war on the side of the Central Powers, while Romanian public opinion sided with the Triple Entente. Carol had signed a secret treaty in 1883 which had linked Romania with the Triple Alliance (1882) and although the treaty was to be activated only in case of attack from Imperial Russia towards one of the treaty's members, Carol was convinced that the honourable thing to do was to enter the war supporting the German Empire.
An emergency meeting was held with members of the government where Carol told them about the secret treaty and shared his opinion with them. The strong disagreement he faced is said to have brought on his sudden death. The future King Ferdinand, under the influence of his wife, Marie of Edinburgh, a British princess, was more willing to listen to public opinion.