The obligations of international humanitarian law are not limited to the attacker; the defender is also required to take steps to protect civilians from harm. The requirement to take precautions against the effects of attack requires the defender to minimize the risk that civilians and civilian objects will be harmed by enemy military operations. At its most basic, it obliges defenders to locate military installations away from civilians. Furthermore, where appropriate, the status of objects should be clearly marked. It is - somewhat counterintuitively - about making it easier for the attacker to select lawful targets by making visible the distinction between civilian objects and military objectives. The increasing importance of digital infrastructure to modern life may make complying with these precautionary obligations more complicated. Maintaining separation between military and civilian networks is challenging as both operate using at least some of the same infrastructure, relying on the same cables, systems, and electromagnetic spectrum. In addition, the speed at which operations against digital infrastructure can occur increases the difficulty of complying with the obligation - particularly if such operations involve a degree of automation or the use of artificial intelligence (ai). This paper sets out the source and extent of the obligation to take precautions against hostile military operations and considers how they might apply to digital infrastructure. As well as clarifying the extent of the obligation, it applies the obligation to take precautions against hostile military operations to digital infrastructure, giving examples of where systems designers are taking these obligations into account, and other examples of where they must.