Putin also expanded on a previous admission that the well-armed forces in unmarked uniforms who took control of Ukrainian military facilities in Crimea were Russian soldiers.
Putin's comments, in a documentary being shown on state TV, highlight the extent to which alarm spread in Russia in the weeks following Ukrainian President Viktor Yanukovych's ouster in February 2014 after months of street protests that turned increasingly violent.
Russian forces took control of Ukrainian military facilities on the peninsula and a referendum on secession was hastily called. The referendum, which was widely denounced in the West as illegitimate, reportedly brought overwhelming support for secession. Russia annexed Crimea on March 19, 2014.
In the documentary, which marks a year since the referendum, Putin says of the nuclear preparedness, "We were ready to do this ... (Crimea) is our historical territory. Russian people live there. They were in danger. We cannot abandon them."
Putin said his plans for a Crimean operation started after Yanukovych fled.
"We never thought about severing Crimea from Ukraine until the moment that these events began, the government overthrow," he said, repeating Russia's contention that Yanukovych was the victim of a coup.
He said he called for a "closed opinion poll" of sentiments among Crimeans about whether to remain in Ukraine. He didn't give details as to how this survey was conducted, but said "it became clear that 75 percent of the general population desired to join Russia."
Russia's Black Sea Fleet is based in Crimea; it retained the bases after the collapse of the Soviet Union under an agreement with Ukraine.
Putin, however, claimed in the documentary that troops weren't sent from Russia into Crimea even though the upper house of parliament had authorised such a move and said the number of forces there never exceeded the 20,000 authorized under the agreement on basing the Black Sea Fleet there.
