Although Schengen was officially part of the EU, the agreement did not apply to all Member States. At first, the United Kingdom rejected its own national borders, preferring to keep its own national borders. Ireland has followed this example in order to preserve its common territory with the United Kingdom. The Schengen Agreement includes two separate agreements that were ratified in 1985 and 1990 respectively. Between them, they abolished border controls and greatly facilitated transit through Europe. The two individual agreements indicated that the Schengen agreements required a degree of mutual trust in immigration and asylum policies, since new residents of one country would then have access to all other Schengen members without a visa and without a passport. However, the harmonisation of immigration policy is in principle the responsibility of the EU institutions and not of The Schengen itself. While Croatia, which joined the EU on 1 July 2013, also has a legal obligation to join the Schengen area, implementation has been delayed. In March 2015, Ranko Ostojia, then Croatia`s interior minister, said his country was ready to join the Schengen area. Croatia invited the EU to carry out a year and a half technical assessment that began on 1 July 2015.  This assessment was positive and Croatia had access to the Schengen information system in January 2017.
 On 27 June 2017, Croatia joined SIS II for law enforcement cooperation.  In December 1996, two non-EU states, Norway and Iceland, signed an association agreement with the countries that signed the Schengen accession agreement. Although this agreement never entered into force, the two countries were part of the Schengen area following similar agreements with the EU.  The Schengen Agreement itself was not signed by non-EU states.  In 2009, Switzerland officially concluded its accession to the Schengen area by adopting an association agreement by referendum in 2005.  Since then, Portugal has put in place several controls along its border with Spain, during the 2004 European Football Championship and Portugal which hosted the NATO summit in Lisbon in 2010. The last checks were temporarily reinstated from May 10, 2017 to May 14, 2017, during Pope Francis` visit to the city of Fetima, Portugal.  Since June 2017[Update], ten urban transport agreements have come into force. However, in light of the November 2015 terrorist attacks in Paris, the interior ministers of the Member States decided, at a meeting of the Council of the European Union on 20 November 2015, “to carry out immediately the systematic and coordinated checks necessary at external borders, including for those who play the right to free movement”.
 Amendments have been made to the Schengen border code to establish systematic checks on the travel documents of citizens of the European Union, the EEA and Switzerland, as well as their family members who play the right to free movement, using databases relevant to the crossing of external borders.  The new regime came into effect on April 7, 2017. The Treaty of Amsterdam, signed in 1997, officially integrated Schengen into the Framework of the European Union as a Schengen acquis. The Schengen acquis includes the 1985 Schengen Agreement, the 1990 Schengen Agreement, as well as various decisions and agreements adopted during the implementation of Schengen.