BY 1985, U2 had slogged their way to pre-eminence, but worrying noises were coming from the inner sanctum. Middle America had been puzzled by the dense impressionism of The Unforgettable Fire, the first album on which the quartet used the studio as a brush rather than canvas. And although Live Aid symbolised their coming of age as one of the most powerful live acts in the world, Bono, underestimating the powers of television (something he'd never do again) feared he'd blown it with The Leap, a gesture which eventually became one of the day's defining moments.
So, in the summer of '85, the band - not for the first or last time - suffered a crisis of confidence which was only broken by Adam Clayton's ultimatum that they record another album and then split. And despite being interrupted by the 1986 Amnesty tour, the resulting sessions produced enough material for a double album (indeed, some of the overflow, B-sides like 'Luminous Times (Hold On To Love)' and 'The Sweetest Thing', constituted their finest work of this period).
Through The Joshua Tree, U2 forged the uncreated (or at least dormant) conscience of rock 'n' roll. Intrigued by writers like Raymond Carver, Bono rapidly matured as a lyricist, exploring all the corrupt beauty of Uncle Sam's dominion, from the Central American atrocities of 'Bullet The Blue Sky' (featuring Edge's first unreconstructed guitar solo) to the road-movie panorama of 'In God's Country'. Even when the band addressed Dublin ('Running To Standstill'), Ethiopia ('Where The Streets Have No Name') or the 1984 miners' strike in Britain ('Red Hill Mining Town'), Reaganomics - and by natural extension, Thatcherism - was the subtext.
Mind you, this wouldn't have meant diddley if not for the white-hot emotional core of the tunes, especially 'With Or Without You', as bittersweet as anything Lou wrote for Nico.
And in terms of sound, Lanois, Eno and the musicians managed to bring focus to the possibilities suggested by their previous collaboration, incorporating elements of gospel ('Still Haven't Found What I'm Looking For') and country blues ('Running To Standstill', 'Trip Through Your Wires') into their hitherto post-punk agenda. It wasn't as gnarled and fractured as their 1986 appearance on TV GaGa had hinted (that would come later, with 'Hawkmoon 269' off the maligned Rattle ... Hum), but it was definitely a new U2.
It's a testament to the strength of the Joshua Tree material that much of it occupied centrepiece status in the live show right up until PopMart, with the band still chasing new levels of meaning in 'Streets' (a flame-on ritual which would remain a drop-dead moment at any gig), 'Bullet' (recast with swastikas in the face of 90s neo-Nazism) and 'Running To Standstill' (on the Zooropa tour, Bono struck an almost heretical chord with his heroin mime routine and "Hallelujah" refrain as the imaginary smack kicked in).
A fair case could be made for at least four other U2 albums being their definitive statement, specifically Achtung Baby's downright grisly explorations of love-as-war, but also Boy's Wildean wonder, The Unforgettable Fire's elemental symphonics, and the dark horse, Zooropa, possibly their most coherent work to date in terms of sustained atmospheres.
But in the end, The Joshua Tree documents a cultural as well as musical watershed, bookended by the band's momentous homecoming gigs at Croke Park in '85 and '87, both of which reflected an unfamiliar bravado at work in the Irish psyche (which, when you think about it, was entirely at odds with the bleak subject matter of many of the band's songs).
After the 1990 World Cup and the subsequent economic recovery, we'd have a whole host of other ghosts to deal with. And so would U2.
* Peter Murphy
ODD FACT
At one point Brian Eno became so frustrated with the amount of time and energy being expended on 'Where The Streets Have No Name', engineer Flood had to physically restrain him from erasing the master tape.

WHAT THEY DID NEXT
Embarked on the tour that ate the world. Appeared on the cover of Time and Rolling Stone. Reached the point of overkill with Rattle ... Hum. Started from scratch (pun intended) with Achtung Baby.

STAR TRACK
'With Or Without You'.

ACE LYRIC LINE
"You got to cry without weeping/Talk without speaking/Scream without raising your voice/You know I took the poison/From the poison stream/Then I floated out of here" - 'Running To Standstill'. The evocation of urban claustrophobia and paralysis echoes everyone from Joyce to Kafka to Delmore Schwartz to Hubert Selby Jr. The fact that the protagonist's means of redemption is heroin only serves to further blacken the mood.

MAGIC MOMENT
In the closing section of 'One Tree Hill' - a lament for Kiwi roadie Greg Carroll, who died in a motorbike accident in 1986 - Bono's vocal goes off the scale and ends up somewhere between blues howl and gospel exultation, an expression of magic and loss that still makes hairs stand up on the back of the neck.

RELATED ALBUMS BY OTHER ARTISTS
The Band's eponymous second album, or a split-screen special featuring the monochrome of Bruce's Darkness On the Edge Of Town and the technicolour of Born To Run.