Bilateral investment treaties (BITs), which provide protection for the investments of foreign investors, are the pre-eminent source of contemporary international investment law (IIL). BITs have recently become controversial, as many of them give foreign investors the right, in specified circumstances, to sue host states before international arbitral tribunals (investor-state dispute settlement or ISDS). However, BITs predate the recent proliferation of ISDS by several decades. This “early era” of BITs, between 1959 (first BIT between West Germany and Pakistan) and 1989 (start of the “boom” in the number of BITs), is scarcely researched, though many legal protections given to investors today can be traced back to these early BITs. With government archives now open, this research aims to lift the veil on the early investment treaty regime on three levels (legal provisions, treaty form, and regime), using case-studies of key Western European states, the first region to “create” these treaties. It aims to explain why these BITs were developed, what key legal protections meant to their architects, and how the number of treaties started expanding. The research helps to resolve the dearth of historical studies in IIL, and is the first to take a comprehensive look at government archives for BITs. It also gives a much needed historical perspective on the original rationale and meaning of BIT provisions, which can in turn help to address the current “legitimacy crisis” in IIL.