He did not remain away for long, sneaking back into Spain and eventually rising to the rank of Comandante and participating in the Battle of Montejurra.

[6] In 1873 or 1874, he secretly returned to Spain and rose to the rank of Comandante in the Carlist Army, and fought in several battles.

"[6][7] Rivero was consistently oppositional to the government of Cuba, and wound up participating in many duels to defend his positions.

[7] Rivero continued to have problems with the law, and started writing from seclusion in his home at the Castillo de Morro.

As director of this newspaper, he dedicated much of his attention and many of his articles in fighting against the repressive and excessive government administration of Captain General Valeriano Weyler.

[6] Due to his support of the new autonomous government, a mass riot of Hispanophiles attacked the offices of Diario de la Marina, targeting Rivero directly.

[6] This happened despite the fact that Rivero was himself, through his whole life, an adherent to a strict Hispanophile and conservative Catholic ideology.

[7] At the end of the War of Independence, Rivero remained in Cuba and continued running Diario de la Marina.

[6] Under Rivero's leadership Diario de la Marina eventually became the most important newspaper in Cuba, and gained the widest circulation of any publication on the island.