The literature of the First World War shapes our consciousness of war itself: nearly a century later, the language, myths and iconography of that conflict underpin an understanding of what war means across much of the world.

It seems appropriate, then, as we prepare for war in Iraq, that George W. Bush should be immersing himself in the words of a British writer from the Great War. His choice from the canon could hardly be more telling. Not for Bush the grimly inspired ironies of Siegfried Sassoon and Robert Graves, nor the poignant, painful questioning of Wilfred Owen. Instead, every morning at dawn, the US President devotes himself to the exhortations of Oswald Chambers, a Scottish evangelist who died while serving as an army chaplain in Egypt in 1917.

Chambers’s little book, My Utmost for the Highest, provides a daily devotional commentary alongside a biblical text. It is uncompromising stuff, “full of spiritual pluck and athleticism” in the writer’s words, advocating absolute devotion to the will of God. That Bush should be reading this before going into battle says much about the religious belief that permeates his Administration, and much about the way the conflict will be fought and interpreted. It is also central to explaining the disquiet of nations with embedded secular political traditions, most notably France, when faced with the most overtly Christian American President of modern times.

For Chambers, Christianity is about submitting to God’s will, putting aside all other considerations. “If the crisis has come to you on any line, surrender your will to him absolutely and irrevocably,” he writes. His is a Christianity of steely assurance. “There is the Great Divide” in the moral world of Chambers and, for that matter, Bush: one route leads towards a “dilatory and useless type of Christian life”; on the other path, “we become more and more ablaze for the glory of God”.

In some ways, Chambers’s mini-sermons hark back to a civilisation before 1914, an age when, in the words of Paul Fussell, the literary historian of the Great War, “one read Hardy and Kipling and Conrad, and frequented worlds of traditional moral action delineated in traditional moral language”. Rupert Brooke would have recognised a kindred spirit in Chambers:

“Now, God be thanked Who has matched us with His Hour

And caught our youth, and wakened us from sleeping.”

For Chambers the enemy was “evil”, religious duty was clear, and Christian soldiers marched onwards in a straight line. “If you allow physical selfishness, mental carelessness, moral insensitivity, or spiritual weakness, everybody in contact with you will suffer.” His simple, stirring homilies are reminiscent of a personal ad that appeared in this newspaper two days after war was declared in 1914. “PAULINE — Alas, it cannot be. But I will dash into the great venture with all that pride and spirit an ancient race has given me ...” I have often wondered whether Pauline kept the cutting, or burnt it.

The evangelical language of preachers such as Chambers increasingly resonates in the words of George Bush. In his State of the Union address, he referred to faith’s “wonder-working power”, a phrase borrowed from a popular evangelical hymn, and invoked “the loving God behind all of love and all of history”. The axis was changed to “evil” from “hatred”, according to a former White House speechwriter, explicitly to make it more “theological”. “We are meeting those challenges because of our faith,” Bush said recently.

The comparisons drawn between Bush’s belief and Tony Blair’s religious commitment are facile. Blair’s religion is private, the result of a lifetime’s interest in theology; that of Bush is all-embracing, public, intimately linked to his decision to give up drinking and thus to his rise to the presidency. The key to Bush’s emotional brand of religion (utterly different again from the restrained Episcopalianism of his father) is what one associate calls the “Goodbye Jack Daniels — Hello Jesus moment”. Bush believes that God put him in the White House. “Beware of giving over to mere dreaming once God has spoken,” is the advice of Chambers. Since the age of 40, it has also been the guiding principle of Bush’s life.

Nothing, of course, should prevent Bush and many other Americans from gaining strength and comfort from their spiritual beliefs. Despite having once used the word in error, Bush is not on a religious “crusade”. But he does believe the Providence that helped him to quit drinking now guides his hand over Iraq. Bob Woodward, in his book Bush at War, described the President “casting his mission and that of the country in the grand vision of God’s master plan”.

As one former White House official told me last week: “If you believe God is on your side, there’s no argument; but argument is the only way to make reasoned decisions.” To frame the conflict in religious terms may turn out to be Bush’s biggest error, alienating many Europeans and stoking Muslim fears that this is a war against Islam being fought in the name of a Christian God. The Churches’ opposition to war, and Bush’s emphasis on faith, have muddied what should be a decision based on rational, humanitarian and geopolitical grounds.

The First World War, more than any other war, helped to undermine the idea of God the Soldier. President Bush should indeed steep himself in the literature of that time, but in the stark, salutary realism of the Great War poets, memoirists and historians.

John F. Kennedy also turned to the First World War during the Cuban missile crisis. He found inspiration not in the Bible, but in Barbara Tuchman’s book The Guns of August, a brilliant depiction of the human misjudgments that led to war in 1914. Bush’s reliance on Chambers (“Rise to the occasion; do the thing”) should be tempered by Tuchman’s secular warning: “History is the unfolding of miscalculation.”