Warsaw-pact forces attacked the country in late August, putting an end to Alexander Dubcek's Prague Spring, and establishing a permanent Soviet military presence which lasted until the collapse of Communism in Eastern Europe.

To send or not to send The 1968 invasion also led to a bitter argument between then UK Prime Minister Harold Wilson and his Foreign Secretary Michael Stewart over whether to send Christmas cards to Soviet Party General Secretary Leonid Brezhnev and other Warsaw Pact leaders that year. According to the released government documents, Mr Stewart argued that "it might be better for none of the aggressors' leaders to receive Christmas cards." But Mr Wilson thought it would seem "petty to change the practice", and reminded the Foreign Office that diplomatic relations were not broken off. After several exchanges of notes, the issue had to be raised at a cabinet meeting, where it was decided that "Ministers should send Christmas cards as they wished." "The Prime Minister therefore decided that he would send cards to the same members of these governments as in 1967."

According to the documents, which were released on the expiry of the 30 year Cabinet Rule, the invasion triggered bitter mutual recriminations over the low numbers and quality of Nato forces in Europe.

"I can only say that this failure to translate recognition of the new Russian capability into specific urgent recommendations for measures to improve our conventional capability could lead us back to quick recourse to nuclear war in case of attack," US General Lyron Lemintzer of Nato's Supreme Allied Command Europe (Saceur) is quoted as saying in the minutes of a Nato Military Committee meeting held in Brussels in September 1968.

"I am certain we all agree this could be a dangerous price to pay for the lack of military prudence."

A Nato report into the implications of the invasion had been removed from the UK records and destroyed for security reasons, but notes of the discussions about the report remained intact and repeated some of its key sections.

"The increased Warsaw Pact capability for initiating an attack, without build-up, gives added emphasis to the need to improve Nato's capability to deal with an attack with little or no warning by some or all of the forces immediately available to the Warsaw Pact," read one paragraph of the 1968 report.

Varied weapons

Baron de Cumont, the Belgian chairman of the Military Committee, said one crucial shortcoming was the lack of standardisation of military equipment within Nato.

"He deplored the way in which some member countries ordered types of equipment, particularly aircraft, which were not compatible with the facilities and equipment available in other countries," the minutes noted.

"He pointed out that the equipment of the Warsaw Pact countries was standardised and the advantages of this had been most apparent in the invasion of Czechoslovakia."

Baron de Cumont also expressed worries about the performance of the North Atlantic Council, Nato's main decision-making body, at a meeting with Ministry of Defence Chiefs of Staff on 10 September 1968.

"One of the main lessons learned had been the reluctance of the NATO Council to take decisions," the minutes said.

They quoted de Cumont as saying that only under heavy pressure from the military committee had the council ordered even minor changes to the preparedness of Nato forces after the invasion of Czechoslovakia.

In November, Nato ministers agreed to improve the alliance's forward positions by upgrading the number of troops and the amount of weaponry. It also vowed to increase the calibre and responsiveness of reserve forces.