Hillary Clinton’s presidential campaign was thrown into turmoil by the leak of emails documenting dirty tactics by the Democratic party leadership. Suggestion of attempts to rig the primary contest in her favour is not merely embarrassing; it is bound to anger supporters of her defeated rival, Bernie Sanders, and deepen the party’s divisions. But this should not be a source of satisfaction for Republicans. They should be equally alarmed by the possibility that the leak came from Moscow — marking a serious escalation in the long term trend of Russian aggression in cyber space.

It is extremely difficult to pin down the source of a hacking attack — especially given the Kremlin’s tendency to outsource such work to criminal groups or pro-government organisations. However, cyber security firms have set out credible evidence to identify the groups responsible for hacking the Democratic National Committee’s network — best known as APT 28 and APT 29. They say that the techniques these groups use, the timezone in which they operate and their previous targets — including the White House and US State Department, eastern European governments and Nato — suggest close links to the Kremlin or Russian security services.

It would hardly be a new development for Moscow to adopt such tactics. As far back as 2007, Estonia suffered crippling attacks on the websites of its banks, government and telecoms networks. Georgian state websites were targeted shortly before the outbreak of war in 2008. Of course, many other governments, the US foremost among them, employ similar methods to gather intelligence and counter security threats.

What is worrying is that Russia is increasingly apt to use its intelligence findings to influence events on the ground. Cold war techniques of bugging and blackmail have new and dangerous possibilities in the era of social media and WikiLeaks. Moscow is combining old-fashioned tradecraft with a willingness to wage wars of disinformation, seeking to muddy the waters and undermine people’s faith in their own country’s institutions and authorities.

This was true in Ukraine, where hackers attempted to corrupt the results of the 2014 elections. It was apparent in Syria, where Russia waged a systematic cyber campaign against opposition groups and NGOs, seeking to influence the flow of information and hide the extent of its military operations. If it is true that Moscow is behind the leak of the DNC’s emails, this represents a far more brazen and audacious attempt to strike at the heart of the US democratic process.

This is an especially dangerous development at a time when there are so many cleavages in western society that can be exploited. Yet it is very hard to identify an effective response.

The US government can hardly protest about a leak that demonstrates Democratic failings. More generally, it is very difficult to put up effective technical defences against such attacks. As for diplomacy, the US has had some success — it struck a deal with China to refrain from cyber espionage that has at least helped. However, this was possible because Beijing was unwilling to be shown up and anxious to preserve its commercial interests. The US has no such leverage over Vladimir Putin. Sanctions against Russia are already in place. As for deterrence, there is only so far rhetoric can go.

The immediate effect of the leak may be to damage Mrs Clinton’s campaign. But this is no cause for Donald Trump to celebrate. It is rather a reminder that this is no time for the US to weaken its commitment to transatlantic security.