Just over three hundred years ago, Anglo-Dutch forces captured Gibraltar during a war over who would be the next king of Spain.
In 1713, the Kingdom of Castile ceded to "the Crown of Great Britain the full and entire propriety of the town and castle of Gibraltar" forever.
That handover, part of the Treaty of Utrecht, has been a thorn in the side of relations between Britain and Spain ever since.
Even though the vast majority of Gibraltarians feel they have their own identity, the Spanish still believe they have a claim to the area they call ‘El Peñon’.
Here we list some of the reason why.
1. National pride
“This is the ‘I wouldn't want a French colony in Plymouth’ variety of argument,” Chris Grocott, a lecturer in economic history at the UK’s Leicester University tells The Local.
In fact, Gibraltar is the only colony left in Europe, something which particularly riles Spanish nationalists.
For Spanish patriots of the 19th century, The Rock was “a totemic issue”, and “a symbol of Spain’s decline,” argues Gareth Stockey, an expert in modern Spain and Gibraltar at the Nottingham University in England.
Under the rule of nationalist dictator Francisco Franco — in power from 1936 to 1975 — relations between Spain and Gibraltar were particularly fraught.
Franco even went as far as selecting August 4th as day to commemorate the loss of Gibraltar: this was day the Gibraltar was taken by Anglo-Dutch forces in 1704.
This pro-Spain sentiment continues to the present day.
In March, Spain's Foreign Minister Jose Manuel Garcia-Margallo said he had "never set foot on Gibraltar" and wouldn't do so until "a Spanish flag is flying on The Rock".
The dispute over the status of Gibraltar even reaches into the royal sphere.
In 2012, Spain’s Queen Sofia cancelled her visit to the UK to take part in Queen Elizabeth’s diamond jubilee celebrations because Gibraltar was making it hard for Spanish fishermen to work.
2. The British have breached the Treaty of Utrecht
Under the Treaty of Utrecht, the Spanish Crown has first right of refusal if Britain gives up its claim on Gibraltar.
This means Gibraltar would, theoretically, automatically come under Spanish rule if the British Crown renounced The Rock.
But in the 1960s, when the United Nations was putting pressure on colonial powers to give up their foreign possessions, Britain sidestepped Spain's right of first refusal.
They did this by granting the people of Gibraltar the right to choose their own future — a decision which saw Gibraltarians voting in 1967 to stay with Britain.
The British have also broken the terms of the Utrecht treaty by expanding the territory of The Rock both on land and sea.
The treaty saw Spain's King Felipe V hand over the "entire property of the city and the castles of Gibraltar, along with its port, defences and fortifications" to the British.
However, Spain has long complained that Gibraltar has expanded beyond its original treaty limits.
While the Gibraltarians claim that fortifications existed along the current border, the status of the isthmus between Spain and the Rock is disputed.
Spain says the building of the airport on this neck of land was illegal and that Britain took advantage of Spain’s civil war to forge ahead with its construction.
At the same time, Gibraltar has extended seawards, claiming the waters around The Rock.
However, the treaty of Utrecht made no mention of any such territorial waters.
This has been the source of many recent disputes between Spain and its diminutive neighbour.
The latest problems between the two stem from Gibraltar’s decision to create an artificial reef off its shore by tipping concrete blocks into the sea.
Gibraltar claim this is being done to boost depleted fish stocks whereas Spanish fishermen say authorities on The Rock are trying to block access to their traditional fishing grounds.
"There are two types of fishing going on in Gibraltar," Grocott tells The Local.
"The British are fishing for ‘fish’ and then there is the Spanish fishing, which is more for ‘seafood’".
This means the Spaniards need to be able to access the sea bed, but with the new concrete blocks in place, they won’t be able to do so.
3. Smugglers paradise
Gibraltar has traditionally been considered a den of smugglers.
Even the original treaty of Utrecht mentions this concern, with authors saying "abuses and frauds" are to be avoided.
The smuggling problem remains to the present day and one that Stockey of Nottingham University calls "a drain on Spanish resources".
Tobacco seizures on the border between Spain and Gibraltar have risen 40 percent in 2013, according to Spain’s El País newspaper.
Other contraband that has passed through Gibraltar into Spain includes heroin and cocaine.
4. Tax haven
Gibraltar is now on the Organisation for Economic Cooperation and Development's "white list" of jurisdictions that comply with global tax rules.
But Spain’s foreign minister García Margallo recently told ABC newspaper that Madrid still intends to crack down on fiscal fraud by Gibraltarians.
An investigation into some 6,700 Gibraltarians who live in Spain but use Gibraltar as their tax residence may be launched.
More importantly, though, Spain wants to turn the screws on companies taking advantage of Gibraltar’s tax laws.
Gibraltar offers a very favourable tax rate of 10 percent, compared to Spain’s 30 percent.
Although the exact figure is unknown, the number of businesses registered on The Rock is thought to be more than 30,000, which is the number of people who live in the territory.
Spain is now considering changing the law so that online gaming companies operating in Gibraltar come under Spain's taxation regime.
"The Spanish government also argues that Gibraltar’s gaming industry is taking advantage of internet infrastructure put in place by Spain," Stockey tells The Local.