Vladimir Putin has admitted for the first time that the plan to annex Crimea was ordered weeks before the referendum on self-determination.
Crimea was formally absorbed into Russia on 18 March, to international condemnation, after unidentified gunmen took over the peninsula.
Mr Putin said on TV he had ordered work on "returning Crimea" to begin at an all-night meeting on 22 February.
The meeting was called after Ukrainian President Viktor Yanukovych was ousted.
Speaking last year, Mr Putin had said only that he took his final decision about Crimea after secret, undated opinion polls showed 80% of Crimeans favoured joining Russia.
The findings of these polls were borne out by the outcome of the referendum on 16 March, he told Russian state TV last April.
Speaking in a forthcoming Russian TV documentary, Mr Putin said a meeting with officials had been held on 22-23 February to plan the rescue of Ukraine's deposed president.
"I invited the leaders of our special services and the defence ministry to the Kremlin and set them the task of saving the life of the president of Ukraine, who would simply have been liquidated," he said.
"We finished about seven in the morning. When we were parting, I told all my colleagues, 'We are forced to begin the work to bring Crimea back into Russia'."
The trailer for The Path To The Motherland was broadcast on Sunday night with no release date announced.
On 27 February, unidentified armed men seized the local parliament and local government buildings in Crimea, raising the Russian flag.
Among them appeared to be regular soldiers without military insignia, who were dubbed the "little green men".
Mr Putin subsequently admitted deploying troops on the peninsula to "stand behind Crimea's self-defence forces".
Timeline: How Crimea was annexed
- 22 February, former Ukrainian President Viktor Yanukovych flees Kiev after violent protests
- 23 February, Russian President Vladimir Putin plans to rescue Mr Yanukovych and annexe Crimea
- 27 February, pro-Russian gunmen seize Crimea's parliament and other key buildings
- 28 February, unidentified soldiers in combat fatigues occupy two airports in Crimea
- 1 March, Russian parliament approves Mr Putin's request to use force in Ukraine
- 16 March, 97% of voters in Crimea agree to join Russia
- 18 March, Mr Putin signs a bill absorbing Crimea into the Russian Federation
The formal annexation of Crimea sparked unrest in eastern Ukraine on 7 April, when pro-Russian protesters occupied government buildings in Donetsk, Luhansk and Kharkiv demanding independence.
A month later, pro-Russian separatists in Donetsk and Luhansk declared independence from Ukraine after unrecognised referendums.
Ukraine responded by launching an "anti-terrorist operation" against them and the region became engulfed in a conflict which has cost at least 6,000 lives and driven more than a million people from their homes, according to the UN.
The Ukrainian government, Western leaders and Nato say there is clear evidence that Russia is helping the separatists with heavy weapons and soldiers. Independent experts echo that accusation.
Moscow denies it, insisting that any Russians serving with the rebels are "volunteers".
Full details of Mr Yanukovych's escape from Ukraine are unclear although Mr Putin spoke of preparations to evacuate him from Donetsk.
"Heavy machine guns were placed there, so as not to waste time talking," he added, with preparations made by land, sea and air.
The documentary, which Russian TV says will be broadcast soon, was made by Andrei Kondrashov, a journalist with state-run channel Rossiya-1.