Netherlands - Belgium - Luxembourg
The 1827 concordat contains one of the last examples of a treaty curse against anyone who dares to infringe it. Pope Leo XII closes it by threatening “the wrath of the Almighty God and of His Holy Apostles Peter and Paul”. In spite of this dire warning, the war and final separation of Belgium from the Netherlands in 1831 made this concordat a dead letter almost as soon as it was signed.
The treaty curse has a long tradition. In Assyria the threat was backed up by a graphic demonstration on an animal of what would happen to any person breaking a treaty. In a vassal treaty with a North Syrian ruler called Mati'ilu, King Assur-nirari VI (753-746 BC) warned him:
Should Mati'ilu break these agreements, his head should be cut off, just as this head of the ram has been cut off. 
Nor was the punishment confined to the ruler. His city was razed, burnt and sown with salt and brimstone to destroy the fertility of the soil. This was the standard punishment among the Assyrians and their neighbours for breaching a treaty. In Deuteronomy 29 22-25 Lord makes a similar threat against anyone who breaks a covenant with Him:
The whole land will be a burning waste of salt and sulfur—nothing planted, nothing sprouting, no vegetation growing on it.
A dim echo of this ancient tradition can be found in the claim at the end of Pope Leo XII's concordat. Anyone who infringes on it "draws upon himself the wrath of the Almighty God and of His Holy Apostles Peter and Paul".
Despite the dire threats, this concordat was a dead letter from the start. However, Vatican influence in the Low Countries has continued by other means. This is especially true of Luxembourg, which came under this concordat after the Congress of Vienna made it a Grand Duchy in personal union with the Netherlands.
The present ruler of Luxembourg, Grand Duke Henri, is said to be a Bailiff Grand Cross of Honour and Devotion of the Sovereign Military Order of Malta. In 2008 he threatened to withhold approval from a planned bill legalising euthanasia.  As a result, parliament rescinded the monarch's right to veto legislation. This also enabled the liberalisation in 2012 of Luxembourg's abortion laws. 
The Church ruled over many European territories, not just the Papal States. There were dozens of prince-bishops in the former Holy Roman Empire, now Germany and some surrounding countries. For instance, the picturesque little town of Dinant in modern Belgium was ruled by Prince-Bishop of Liège. Naturally, where a cleric reigned, a concordat between the pope and the ruler was not necessary.
King William I of the United Kingdom of the Netherlands found himself in an increasingly insecure position. Because he favoured the Dutch language and Calvinism of his Northern Provinces he was viewed with suspicion by his Catholic French-speaking subjects in the Southern Provinces. This concordat, concluded shortly before they rebelled against him, looks like an attempt to buy the support of the Church.