Today, the Schengen zone comprises 26 countries, including EU non-member countries Iceland, Liechtenstein, Norway and Switzerland. The only EU members outside the passport-free area are Cyprus, Ireland and the UK, while Croatia, Bulgaria and Romania are still waiting to join.

The Schengen zone spans an area of 4.3 million square metres with a population of more than 420 million, who can cross the internal borders of member countries without being subjected to border checks. The same pertains to cargo, which has boosted free movement of goods as a major pillar of the single market.

Upon the biggest expansion of the border-free area to date, land borders were the first to fall on 21 December, 2007. Festivities were held in Slovenia and East European countries as border gates were lifted at new internal borders of the enlarged Schengen zone.

Border checks were also abolished at ports on routes between member states with passport and luggage controls on internal Schengen flights at airports scrapped on 30 March the next year, as flight schedules switched to the summer timetable.

Meanwhile, Slovenia introduced stricter checks on its southern border as the country assumed the task of securing 718 kilometres of the external Schengen border - 670 kilometres on its land border with Croatia and 48 kilometres at sea.

Even though the Slovenia-Croatia border had not been determined until the arbitration tribunal declared the award in June this year, this had not been a major problem on the ground until the migration crisis in 2015 when Slovenia put up fence at sections of the border to Croatia's protests.

Efforts are now under way to implement the border award, which Croatia opposes. Once the external border is moved to Croatia's border with Serbia, Bosnia-Herzegovina and Montenegro, Slovenia will be surrounded only by Schengen countries and it will no longer be in charge of external border checks.

This is if Schengen survives the biggest crisis to date. Prompted by the massive migration wave in 2015, many Schengen nations reinstated border checks to curb the inflow of migrants and refugees.

Even though meant as a temporary measure, Austria, Germany, Norway, Denmark and Sweden continue to conduct internal border checks and do not plan to scrap them in the near future. The measure is also being preserved by France due to the threat of terrorism.

The migration crisis exposed deficiencies in external border controls. The sheer mass of migrants, coupled with the valid asylum legislation, meant excessive burden on the countries on the external border, in particular Italy and Greece, which face the pressure of migrants on its sea borders.

The European Commission has been trying to solve the issue with updates to the Schengen Code to enable border checks on internal borders to be extended for up to three years in the event of predictable serious threats.

Slovenia has been opposing ongoing checks on the border with Austria arguing that they were unwarranted, because only a handful of illegal migrants crossed the border from Slovenia.

Slovenia has also argued that the measure should be proportional and applied selectively where absolutely necessary, and that it needs to rest on credible and objective criteria.