The concept of mutual solidarity between EU Member States is guaranteed by some provisions of primary law, including the preamble to the TEU and Articles 2 and 3 TEU. It is also enshrined in Article 122 TFEU, which allows the EU to grant financial aid to a Member State in difficulty. Linked to the principle of mutual trust developed by the case law of the Court of Justice (especially Opinion 2/13), solidarity finds itself at the very foundation of EU integration.