Tactical media do not demand artistry. Any craft or art is additional to the primary disruptive purpose of a cultural practice whose energies are directly engaged in conflict, subversion, sabotage. The original photocopied handout the MacLibel Two gave passersby in London's Tottenham Court Road is incidental to the global media blitz that resulted from their decision to defend themselves for two years against Macdonalds' stellar legal team. Tactical media don't even question the importance of art. If art could be used as a weapon, it would be, and is used alongside video, junk mail, phony advertising copy, computer hacking and fly posting. It's just that, in the age of the media, the art gallery is a rather blunt, rather slow weapon. You wouldn't want to rely on it for its efficacy in urban guerrilla warfare.

Which is the quandary facing Narelle Jubelin in her elegant installation in the re-opening show at the Liverpool Tate. On a bench to one side of the more visible components of the piece lie the volumes of court records for the Ploughshares Four, Joanna Wilson, Lotta Kronlid, Angela Zelter and Andrea Needham, who in January 1996 broke into Hawker Siddley's Lancashire plant to destroy key equipment on a jet destined for sale to Indonesia and use in East Timor. The women hid nothing, admitted everything, and explained that the moral obligation to destroy the plane was their last resort when all other remedies failed. They were gleefully acquitted in July the same year amid storms of media coverage.

One of the central planks of art practice over the 80 or 90 years since Duchamp's Fountain has been research into the boundaries of art, into what makes a work art, and how far the definition can be pushed before it begins to lose meaning. Even that very moment of the loss of meaning of the word 'art' has become a subject for art, the moment of the immaterial, the informe, the senseless. Is this the beginning of the postmodern or the dying gasp of modernism's autonomous art object? Should anyone care? The Ploughshares Four may well, but in their spare time. In place of art, they have embraced action, and tools not unfamiliar for a thousand years of assaults on art and technology from the iconoclasts to the Luddites. There is something deeply satisfying in realising how much damage you could do to one of the world's most sophisticated weapons with a simple toolkit of hammer, wrench and crowbar, a profound metacritique of the cult of engineering. Even more, they recognised that the institutions of the law are themselves performance spaces in which the rituals of self-congratulation among transnational corporations can and must be countered when the audience for legal spectacle seizes the stage. The use of the courts as media was perfected by the Greenham peace women. Case number T961301 in the Liverpool Crown Courts is a delightful addition to that new tactical expropriation of legality for rebellion against the present.

Jubelin's problem is to make a work that is as successful as that, and she fails. But her failure is itself the centre of the gallery work's value, and that value is high. The major part of the installation is made up of place settings at three of those anonymous white tables that you find in airport coffee bars, furniture's equivalent to muzak, furniture designed for invisibility, the vacuous context of consumption purified to the point of absolute presence. On these fetishistically vacant surfaces she has arranged cutlery and images of cutlery from the design studios of the UK, knives curiously weighted, fat blades and stubby handles, forks like cleft spoons, spoons with tiny cups and extended handles as if designed for supping with the devil. Many of these were made for British Embassies, to serve less as eating implements than as advertisements for British technologies, manufacture and design industries. They are, in this sense, understandably clumsy for eating because their prime function is to communicate, not least to communicate a method of eating that does not involve such heathen feeding technologies as chopsticks and fingers. Their uselessness communicates civilisation in those bastions where the United Kingdom advertises itself in all its fading imperial glory.

What turns this display into art is the subtlety of the connection between the embassy dinner service and the Ploughshares Four. If the ambassadorial role is in the first instance to communicate presence, the secondary one is to facilitate trade. After all, in our days, communication has become a strange beast. The communication industries tend towards uncommunicative spectacle, vast, pointless and ideologically transparent blockbusters, gameshows and billboards. The bulk of communication, real communication, between adults on the global scale is conducted in terms of the circulation of cash. The function of the embassy in this is to underwrite, guarantee, facilitate and perhaps most of all to ensure that trade communication is restricted entirely to the fiscal domain, with as little seepage as possible into areas of the aesthetic and the ethical. It is not that diplomacy manufactures secrecy, but that it operates to restrict communication between cultures to the level of deals. And no deal as carefully shuns the limelight as an arms deal.

Jubelin's body of anti and postcolonial work over the years feeds into the research for this installation. The webworking of global connectivity is centrally engaged in business, not democracy or nomadology. In this mesh of transnational capital, the invisible component of the work is central: the damaged Harrier jet. The task of the embassy as broker is to ensure that no-one, from Preston to Jakarta, ever has to utter the meaning of the jet, its function, its goal, the destruction of civilian uprising in East Timor. Instead, the plane appears as a cipher in diplomatic rodomontade, as design, technology, manufacture, excellence, and finally as cash tied to other cash: arms, dams, aid, influence. You can visualise the jet's invisibility as a wireframe diagram, whose shimmering outlines indicate not the instruction algorithms of computer-aided manufacture but the lacework of a planetary networked economy. For this reason, the physicality of the jet has to remain outside the gallery, abstracted, conceptualised, a tissue of discretions.

Or this art would not be art. What differentiates Jubelin's installation from the tactical media blast of the Ploughshares Four is the physicality of their intervention. At the site of their non-crime they left handbills and tools decorated with peace and anti-genocide slogans so that at the least the workers clearing up after would get the message. There is a disarming modesty about this courageous conspiracy to end silence. Jubelin's achievement is not that she has made a work in any way the equal of that, but precisely that she has not: that in her intervention, she points up a limit point at which the gallery, however politicised, can never equal the urgency of the tactician. On the other hand, it points towards what art can do, as a slower and more strategic practice: remembering what has been achieved and allowing its savour to penetrate through time into the emerging future.