Asked if Russia was prepared to bring its nuclear weapons into play, Putin said: "We were ready to do it. I talked with colleagues and told them that this (Crimea) is our historic territory, Russian people live there, they are in danger, we cannot leave them.
"It wasn't us who committed a coup, it was the nationalists and people with extreme beliefs."
But this was the worst-case scenario, he added, in the documentary broadcast on state-run channel Rossiya One. "I don't think this was actually anyone's wish -- to turn it into a world conflict."
Russia formally annexed Ukraine's Crimean Peninsula last March
, after Ukraine's pro-Moscow President Viktor Yanukovych was ousted following street protests that turned bloody
. Russia called his ouster a coup by radical Ukrainian nationalists.
Before Crimea was formally absorbed by Russia, unidentified armed men had taken control of its administrative buildings and key military sites. A referendum was held on March 16, 2014 -- a year ago to the day -- on secession from Ukraine.
Ukrainian officials at the time denied there was any threat to Russian citizens in Crimea.
On Monday, Russia's Defense Minister Sergey Shoigu said Putin had ordered Russia's Northern Fleet and other military units to be placed on full combat alert for snap checks, state media reported.
"The main aim ... is to evaluate the capabilities of the Northern Fleet to fulfill tasks on ensuring Russia's military security in the Arctic," Shoigu is quoted as saying by Russia's Tass news agency.
The exercise will involve 38,000 military personnel, 41 warships, 15 submarines and 110 planes and helicopters, Shoigu said, according to Tass.
In the documentary, titled "Crimea: Way Back Home," Putin said Moscow had had no choice but to act.
"Crimea isn't just any territory for us, it is historically Russian territory," he said.
Crimea, a Black Sea peninsula that has been part of Ukraine since 1954, has a majority Russian population and strong cultural and historical ties to Russia. It is also home to Russia's Black Sea fleet, at the Sevastopol naval base.
But Putin said he had never thought of "dismembering" the peninsula from Ukraine until Yanukovych's ouster.
"We cannot leave this area and the people who live there to whims of fate, to let the people of Crimea be thrown under the wheels of this nationalist bulldozer," he said.
"I set certain tasks. I did say what and how we should do, but immediately stressed that we would do so only if we are absolutely convinced that the people who live in the Crimea want it."
The vast majority of people who voted in the referendum approved secession from Ukraine. But the vote, staged while armed men controlled the region, was dismissed as illegal by Ukraine's then-interim government in Kiev, the European Union and the United States.
The West responded with financial sanctions against selected Russian figures and Crimean secessionist leaders.
German Chancellor Angela Merkel, meeting with Ukrainian President Petro Poroshenko in Berlin on Monday, reiterated that Europe does not consider Russia's annexation of Crimea to be legal.
"It is important that we work for a peaceful solution, and we will not give up before the full sovereignty and territorial integrity of Ukraine has been restored," she said. "This not only includes Crimea but also now in our daily work the areas around Luhansk and Donetsk."
Unrest broke out in Ukraine's eastern Donetsk and Luhansk regions following Russia's absorbing of Crimea. A fragile truce is currently in place
but the conflict between pro-Russia separatists and Ukrainian forces there has claimed thousands of lives.
Merkel warned that the European Union was ready to impose more sanctions if necessary against Russian interests to help ensure that a peace plan hammered out in Minsk, Belarus, last month is fully implemented.
Putin: U.S. 'puppeteers' were behind coup
Putin told the documentary-makers he was certain the United States was behind the ouster of Yanukovych, which Moscow views as an illegal armed coup.
"Formally, the opposition was primarily supported by Europeans, but we knew very well ... that the real puppeteers were our American partners and friends. It was them who helped prepare nationalists (and) combat troops," he said.
He also said that sanctions should have been imposed against those who orchestrated what he called a coup, rather than those involved in Crimea's annexation.
Putin insisted in remarks to reporters last month that there was no chance Crimea would be returned
And it appears Russia is taking no chances.
In the documentary, Putin said Russia's Bastion high-precision coastal missile defense systems had been deployed to Crimea to protect the territory -- "in such a way that they were seen perfectly well from outer space."
This can only be done following a decision by the supreme commander in chief, he said. "So, at a certain point, we deployed these coastal systems Bastion to make it clear that Crimea was under safe protection," he added.