Recent trends in Democratic Peace theory have called into question the orthodox ‘separate democratic peace’ position that liberal states are peace-prone only in relations with other liberal states. This article seeks to recast the bases and scope (or parameters) of the relationship between liberalism – primarily left-liberalism – in domestic politics and peace-proneness in foreign affairs, to the effect that whilst this is manifest more broadly than conventionally understood, it is far from universal or undifferentiated. Whilst liberal ‘norms’ – as indicators of the legitimacy of force – are an important factor in determining when liberals will and will not support the use of force, liberals are also shown to have a higher threshold for the use of force than other groups on the mainstream domestic political spectrum (usually to the Right), indicating greater unwillingness to use force. The cases examined are reasonably neutral, not cherry-picked, in that they are drawn from the three major conflicts of the twentieth century and, as a starting point for the examination of liberal imperialism, Gladstone's Second Ministry (1880–1885).

Whilst the above findings are presented as highly significant for understanding the character of liberal peace-proneness and an advance on the separate democratic peace position, the argument is necessarily qualified in an effort to reflect the political complexities of the phenomenon and the limitations of liberal norms as an explanatory factor. It is, however, these very qualifications that put the politics back into the research agenda and connect the knowledge claims to a wider body of academic scrutiny which, ultimately, should lead to a fuller understanding of the relationship between liberalism, democracy and peace.