The Czech and Slovak governments have accused Russia of rewriting history after Russian state TV broadcast a documentary about the 1968 Soviet-led invasion of Czechoslovakia.
The programme, aired on 23 May, argued the invasion was aimed at preventing a military coup backed by the West.
The Czech government has summoned the Russian ambassador and Slovakia has protested about the documentary.
The August 1968 invasion crushed months of reforms known as the Prague Spring.
Led by Alexander Dubcek, the Czechoslovak Communist Party had relaxed press censorship and pushed for "socialism with a human face".
But the Soviet leadership sent in 2,000 tanks and a 500,000-strong force, including soldiers from other Warsaw Pact countries, prompting clashes and the deaths of more than 100 people and the return of hard-line Soviet rule.
Warsaw Pact - Pages Declassified, broadcast on state-run Rossiya 1 TV, argued that the Soviet-led military alliance was a purely defensive organisation in the face of an "aggressive" Nato alliance.
The programme claimed a Nato-backed "armed coup" was being planned under the cover of the "legend of peaceful civilian uprising with the romantic name of the Prague Spring".
Excerpts from pro-Soviet propaganda films made shortly after the the invasion were included, along with testimony from Russian Communist MP Yury Sinelshchikov who was in the Soviet military at the time.
Czech Foreign Minister Lubomir Zaoralek summoned Russia's Prague ambassador Sergei Kiselyev to complain about the broadcast as well the inclusion of several Czechs on a list of 89 EU politicians and diplomats banned from entering Russia.
The Slovak foreign ministry described the programme as a "false and truth-distorting work" that used "misrepresentations and old ideological cliches", pointing out that Russia had in the past officially apologised for the invasion and subsequent occupation.
Communism finally fell in 1989 in Czechoslovakia and the federal state eventually split into two in 1993.
The government in Bratislava said the decision to broadcast the documentary damaged traditionally good relations between Slovakia and Russia. It also pointed out that the Slovak government had vigorously refused what it said were attempts to revise the history of World War Two in a way that cast doubt on the role of the Red Army.
The spat comes on the eve of talks between Slovak PM Robert Fico and Russia's President Vladimir Putin and Prime Minister Dmitry Medvedev.
Slovakia has helped the EU supply gas to Ukraine during its conflict with pro-Russian rebels through a system known as reverse flow.