In the 1850s (after the Crimean War), Focşani grew in importance as the center of activities in favor of the union between Wallachia and Moldavia (the Danubian Principalities), leading to the double election of Alexander John Cuza in Iaşi and Bucharest. Following this, it housed a Central Commission regulating the common legislation of the two countries, as well as the High Court of Justice. Both institutions were disestablished in 1864, when the Romanian Principality was founded as a unified state. Focşani's role in the forming of the modern Romanian state is immortalized in the Union Square Obelisk.
On December 30-31, 1881, following the impact of Zionism on the Romanian Jewish community, the First Congress of all Zionist Unions in Romania for the promotion of the colonization of Eretz Israel was held at Focşani. It was attended by 51 delegates, representing 32 organizations, two press editors, three newspaper reporters and important guests. This 1881 Congress, the first ever held, 16 years before the World Zionist Organization's First Zionist (held in Basel), had a major influence on the Romanian Jews, and its proceedings also became known outside the borders of Romania.
In 1917, during the Romanian Campaign of World War I, Focşani and Galaţi were part of a line of fortifications known as the Siret Line. An armistice was signed in the city on December 9, 1917, between the Kingdom of Romania and the Central Powers.
In 1944, during World War II, Focşani was supposed to be part of the fortified Focşani-Nămoloasa-Galaţi line, where 9 elite divisions were preparing to resist the Soviet Red Army's advance after the Battle of Târgul Frumos. However, due to the turn of events on August 23, 1944 (see Romania during World War II), this never materialized.