The report does not put the whole blame on either country
The war in Georgia last year was started by a Georgian attack that was not justified by international law, an EU-sponsored report has concluded.
However, the attack followed months of provocation, and both sides violated international law, the report said.
Russia said the report delivered an "unequivocal answer" on the question of who started the conflict.
But Georgia said the investigation proved that Russia had been preparing for war all along.
The report said about 850 people were killed in the August 2008 war, and that more than 100,000 fled their homes, about 35,000 of whom are still displaced.
It was commissioned by the Council of the European Union, and written by Swiss diplomat Heidi Tagliavini, with the help of 30 European military, legal and history experts.
The conflict erupted on 7 August 2008, as Georgia shelled the breakaway region of South Ossetia, in an attempt to regain control over it. The previous months had seen a series of clashes.
Russian forces quickly repelled the assault, and pushed further into Georgia.
The conflict lasted for five days before a ceasefire was agreed. Russia pulled back, but built up its military presence in both South Ossetia and Abkhazia.
"The shelling of Tskhinvali (the South Ossetian capital) by the Georgian armed forces during the night of 7 to 8 August 2008 marked the beginning of the large-scale armed conflict in Georgia," the report says.
It adds later: "There is the question of whether [this] use of force... was justifiable under international law. It was not."
It also says Georgia's claim that there had been a large-scale Russian military incursion into South Ossetia before the outbreak of war could not be "sufficiently substantiated", though it said there was evidence of a lower-level military build-up.
The report states that while Russia's initial actions in fighting back against attacks on its personnel in South Ossetia were justified, its subsequent actions, in pushing far into Georgia proper "went far beyond the reasonable limits of defence" and was "in violation of international law".
"Furthermore, continued destruction which came after the ceasefire agreement was not justifiable by any means."
Fighting damaged buildings in the South Ossetian capital Tskhinvali
Given the European Union's relations with Russia have improved compared to a year ago, the EU may welcome the report itself, but may want to distance itself from the content, says the BBC Brussels correspondent Dominic Hughes.
EU countries said in a statement the report was not about apportioning blame, but they hoped it could "contribute toward a better understanding of the origins and the course of last year's conflict".
Russia has recognised South Ossetia and Abkhazia as independent and has vowed to protect them.
However, Georgia and the vast majority of the international community still views them as part of Georgia, and the report's author said Russia's recognition "must be considered as being not valid in the context of international law, and as violations of Georgia's territorial integrity and sovereignty."
Aid agencies say a refugee crisis continues in the region, with the Russian-backed authorities in South Ossetia refusing to allow tens of thousands of ethnic Georgians back to their homes in the region.