They say that religion and politics don’t mix, but it’s impossible to separate the two when the Pope and the American President have gone head-to-head over the war in Iran. In a social media post, President Trump accused Pope Leo of being weak and advised that he should “focus on being a Great Pope, not a Politician”. Pope Leo responded by insisting that he’s not a politician, but that the message of the Gospel, “‘Blessed are the peacemakers’, is a message that the world needs to hear today”.

This confrontation has catapulted the Pope onto the front pages of the world’s media, but he’s not the first modern pope to speak out against war. In 2003, when then Cardinal Ratzinger, later Pope Benedict XVI, was asked to comment on the Iraq war, he said that “There were not sufficient reasons to unleash a war against Iraq.” He went on to ask “if it is still licit to admit the very existence of a ‘just war’.”

St Augustine gave a Christian interpretation to the idea of the just war in the early 5th century. He argued that, terrible though war always is, it is sometimes necessary to defend the innocent and preserve peace. However, it must seek the future well-being of the enemy, and be free from the lust for power or desire to dominate.

These ideas were developed by St Thomas Aquinas in the 13th century, and they continued to shape western politics and international law long after Christianity ceased to be a major political influence. However ineffectual it might sometimes have become in the heat of battle, just war theory provided a restraining influence on the waging of war, especially with regard to the need to avoid the intentional targeting of non-combatants. Today, the nature of modern weapons and the bombing of densely populated areas means that civilian casualties, including children, usually far outnumber military deaths.

This is the context in which the Catholic Church’s opposition to war must be interpreted. Pope Leo is continuing a tradition set by all modern popes since the 1960s. In his Palm Sunday address, he quoted the prophet Isaiah when he said that Jesus “does not listen to the prayers of those who wage war, but rejects them, saying: ‘Even though you make many prayers, I will not listen: your hands are full of blood’.” This is religious language, but it holds politicians accountable for shedding innocent blood. How could it do otherwise, when Christians worship a crucified God?