Worster worked as an immigration lawyer in the United States before moving to the Netherlands, where he’s been teaching at The Hague University of Applied Sciences for the past 16 years. ‘Diplomatic assurances have been used for decades to help ease the transition from deporting people from one state to another. While I was working in the field of immigration law, I became interested in the promises states make and how diplomatic conversations create international law.’

What do these diplomatic promises look like?

‘They are tailor-made for each situation. That was a challenge during my research. If someone could be tortured after deportation, the promise could be not to torture the expelled person. Other states might even allow you to bring in monitors and check in on the well-being of the deported. It can get quite explicit. On the other hand, one of the assurances I found said, “Don’t worry, he’ll be fine”. The other state accepted that even though it didn’t create any legal obligation. These promises sometimes do create legal obligations, and at other times, they are merely diplomatic niceties. A treaty can be binding yet add little new substantive constraint on the states, while a unilateral promise can create binding obligations under international law. I had to determine during my research if these diplomatic conversations qualified as treaties or as bilateral promises.’

How did you get access to these diplomatic conversations?

‘Finding the agreements was quite a big challenge. I ended up locating about a 100 of them covering 50 different countries. Some are posted on ministry websites, and I also found documents on WikiLeaks. The agreements are incredibly diverse. Some look like formal agreements, including signatures, whereas others were purely verbal.’

Why are some of the agreements less transparent than others?

‘At times, the agreements were clear, but they’re mostly deliberately vague for political reasons. There is a big risk of misuse in those grey areas. For example, we have recently seen a deal between the United States and El Salvador to expel people to prison. However, I haven’t seen that agreement, except for a declaration on X. If states are willing to be clear on record about such agreements, that is helpful. The deported individual and the courts are then aware of what’s happening. If it’s clear that it’s a binding agreement, the risk of not upholding it and potentially mistreating someone also becomes apparent. When it’s vague, states aren’t quite sure if they have to comply, and people don’t know what rights they have. Courts will do their best to make informed guesses, but it remains difficult.’

Do you not have the right to know why and under what circumstances you are being deported?

‘You have the right to a process, even just to establish that the right person is being deported. But that right is not always honoured. Sometimes people are expelled without due process. The question remains what kind of access individuals have to records about agreements. Sometimes, records contain national security secrets, and even the judge may not have access. That creates a highly problematic process. Even though, in principle, it’s about human rights, it’s ultimately about agreements and relationships between states.’

How could courts get better access?

‘Under human rights law, you cannot expel someone when there is a high risk of human rights violations. You can make agreements to prevent that, but we don’t know if those agreements are legally binding. If the court doesn’t have access to the agreement, it could simply demand a copy. If access is denied and it remains unclear whether human rights will be violated upon expelling someone, the court can decide not to allow deportation. States can have valid reasons to expel someone, for example, if they have committed crimes. But courts have leverage as protectors of human rights. I hope my research helps courts addressing the lack of clarity in these assurances. My thesis shows that some diplomatic assurances are law, and some are not. Even when they are law, it can range from thin to genuinely constraining law. In my thesis, I aimed to create a roadmap to help courts separate the binding from the non-binding assurances and the empty from the non-empty assurances.’