In this centenary year of the outbreak of the First World War, it is fitting that the National Theatre's hit show War Horse should have two parallel productions; this presentation, which is touring the UK and Ireland, and another installed at the New London Theatre.

Based upon former Children's Laureate Michael Morpurgo's acclaimed novel, Nick Stafford's play has become an international phenomenon since its premiere at the National Theatre itself in 2007. As this latest production attests, it is worthy of every accolade it has received.

Morpurgo's intelligent, heartfelt narrative follows the journeys through the war of both Joey, a fine Devonshire horse, and his young master, Albert. The NT's staging of it is deservedly famous for its extraordinary, life-sized horse puppets. However, the production's greatest achievement is that its grand theatre technologies (including unusually effective stage projections) always serve, rather than overwhelm, the story.

Morpurgo's tale, in which Joey is a rare equine survivor of the folly of cavalry charges into barbed wire and machine-gun fire, opens into a German experience of the war which is every bit as brutal and pitiless as that of the British. The pathos and reconciliatory intent of the novel are realised powerfully on a cleverly minimalist set by way of painful ensemble scenes, touching songs (by John Tams) and brilliant moments of horse choreography.

War Horse mourns the loss and suffering of both man and beast, on both sides of the "war to end all wars". A work of humane, gorgeously structured, visually spectacular storytelling, it is a near perfect piece of popular theatre.

As such, one is surprised that it has not joined the likes of Blackadder and Oh! What A Lovely War on Westminster education secretary Michael Gove's tawdry little list of "left-wing" artworks which he has said fail to depict the slaughter on the Somme with due respect for "virtues such as patriotism, honour and courage".

There is human tragedy on a much smaller, but nonetheless agonising, scale in the Royal Lyceum's production of what is, arguably, Eugene O'Neill's magnum opus, Long Day's Journey Into Night. This play - which had its UK premiere at the Lyceum in 1958, and won the Nobel Laureate a posthumous Pulitzer Prize - paints a thinly veiled picture of the impossibly anguished relations which prevailed within his family.

The Tyrones (who take their name from the Irish county from which the O'Neills migrated to the United States) are blighted by addictions (to alcohol on the parts of the men; to negligently prescribed morphine in the case of Mary, O'Neill's mother).

Their Connecticut summer home reeks with the odours of unfulfilled potential, blame, guilt, mutual animosity and self-hatred. Mary drifts into a drug-induced haze. The menfolk (one-time leading actor James, and his sons Jamie and Edmund, the latter of whom has just been diagnosed with tuberculosis) drink themselves into recriminatory bitterness. Yet there is a sense that, in spite of everything, these ill-fated people still love and, crucially, forgive each other.

The enduring success of this play lies, partly at least, in its universally recognisable human relations (albeit that, in O'Neill's family, they are on a titanic scale). It is, however, a drama which can easily flounder on the stage. The beauty of director Tony Cownie's nicely balanced, excellently acted production (blessed, as it is, with a cleverly naturalistic, yet bleakly abstract set by Janet Bird), is that it charts a careful and sensitive course between the twin dangers, inherent in this almost three-hour long play, of languid resignation and bathetic hyperactivity. Consistently true to O'Neill's conflicted autobiography, it is a very fine production of a modern American classic.