The Ulster Covenant wasn’t the only thing that led to the birth of Northern Ireland as part of the United Kingdom, but it was one of the main ones.
Half a million Irish Protestants, fearing that “home rule was Rome rule”, threatened to use any means necessary to oppose it.
No wonder unionists and loyalists regard it as a centenary worth marking. And tomorrow there will be re-enactments and marches in Belfast to celebrate. There will be a “family fun” day at the Stormont estate. The Orange Order is expecting 30,000 participants. Many more will line the streets.
And that’s where it’s feared the fun could come to an abrupt end. Tensions have run high around this year’s marching season. Both loyalist and republican rioters have clashed with the police, most seriously in the area around St Patrick’s Catholic church on the Donegal Street where a loyalist band stopped and allegedly played sectarian music during the summer. These things count for a lot when tempers are already boiling over.
You may not have learned about The Ulster Covenant in school but it's a key document in the foundation of the modern United Kingdom. A century ago today half a million Irish men and women signed a document telling London they would use any means necessary to stay part of the UK, to oppose Irish Home Rule.
In 1912 the Liberal Government needed the support of Irish nationalist MPs in Westminster. In return the nationalists wanted home rule; today we'd call it devolution. Irish unionists knew it was a step towards an independent Ireland. Those in the north east of the country – inspired by William Craig and Edward Carson – queued up to sign the Covenant.
And tempers are running high. Loyalists feel the Parades Commission is trying to stifle their culture at the behest of republicans. Republicans say they shouldn’t have to put up with sectarian parades in their areas.
There are suspicions that paramilitary groups on both sides are looking to exploit the tensions to bolster their own positions.
On both sides leaders have called for respect, dignity and peaceful demonstration to be the guiding principles for tomorrow. But the new Northern Ireland Secretary Theresa Villiers was right when she said: “I think everyone is nervous about how it will go on Saturday.”