It is not always easy to determine when an invasion has become an occupation and whether or not the law of occupation could already be applied during the invasion phase. In this regard, two main positions are usually put forward in legal literature. Generally it is held that the provisions of occupation law only apply once the elements underpinning the definition set out in Article 42 of the 1907 Hague Regulations are met. However, the so-called ‘Pictet theory’, as formulated by Jean S. Pictet in the ICRC’s Commentary on the Geneva Conventions, proposes that no intermediate phase between invasion and occupation exists and that certain provisions of occupation law already apply during an invasion. Three experts in the field of occupation law have agreed to participate in this debate and to defend three approaches. Marten Zwanenburg maintains that for determining when an invasion turns into an occupation the only test is the one set out in Article 42 of the 1907 Hague Regulations, and therefore rejects the ‘Pictet theory’. Michael Bothe, while also rejecting the ‘Pictet theory’, argues that a possible intermediate situation between invasion and occupation, if there is any at all, would be very short and that, once an invader has gained control over a part of an invaded territory, the law of occupation applies. Finally, Marco Sassòli defends the ‘Pictet theory’ and argues that, in order to avoid legal vacuums, there is no distinction between an invasion phase and an occupation phase for applying the rules of the Fourth Geneva Convention.