This is, of course, often done because it functions as a convenience but it also carries wide-reaching implications. In his essay Ethnicity without Groups Roger Brubaker writes

"Group" functions as a seemingly unproblematic, taken-for-granted concept (...) As a result, we tend to take for granted not only the concept "group", but also "groups" – the putative things-in-the-world to which the concept refers. (...) This is what I will call groupism: the tendency to take discrete, sharply differentiated, internally homogeneous and externally bounded groups as basic constituents of social conflicts, and fundamental units of social analysis. In the domain of ethnicity, nationalism, and race, I mean by "groupism" the tendency to treat ethnic groups, nations and races as substantial entities to which interest and agency can be attributed. (...) I mean the tendency to represent the social and cultural world as a multichrome mosaic of monochrome ethnic, racial or cultural blocs.

What Brubaker refers to is to imagine such things as the Franco-German war of 1871 or the Middle Eastern conflict as result of an inherent group conciseness of Frenchmen and Germans and of Palestinians and Israelis that stems from these groups being collective historical actors with a will of their own rather than a collection of individuals who frame themselves and each other within such a matrix.

What Brubaker means is the following

We need to break, for example, with the seemingly obvious and uncontroversial point that ethnic conflict involves conflict between ethnic groups. I want to suggest that ethnic conflict – or what might be better called ethnicized or ethnically framed conflict – need not, and should not, be understood as conflict between ethnic groups [emphasize original].

Rather, we need to understand such categories as ethnic or other groupist terms as something invoked and constructed by historical actors. It is these actors who cast ethnic, racial or national groups as the protagonists of conflict, of struggle. In fact, these categories, while essential to the actors casting them, referencing them, are in themselves a construct, a performance.

Brubaker:

Ethnicity, race, and nation should be conceptualized not as substances or things or entities or collective individuals – as the imagery of discrete, concrete, tangible, bounded and enduring "groups" encourages us to do – but rather in relational, processual, dynamic, and disaggregated terms. This means thinking of ethnicity, race, and nation not in terms of substantial groups or entities but in terms of practical categories, cultural idoms, cognitive schemas, discursive frames, organized routines, institutional forms, political projects and cognitive events. It means thinking of ethnicization, racilization and nationalization as political, social, cultural and psychological processes

The gist of it is that in order to gain a greater understanding of history we must think of groups and their group identities not as something a priori, not as something that has an in-the-world-substance but rather as something that is the product of a process and has an ever shifting connotation, content, and consequence. What it means to be German, French, Palestinian or Israeli is not predetermined by characteristics inherent and unchanging to this group moniker but the product of a historical, political, and social process. It is, in fact contingent upon such processes.

So rather than framing our historical narrative centered around groups such as the Germans, the French etc. as the actors of history, we need to view these group identities as products of the historical process. Rather than essentializing what it means to be part of a group existing independently from its members, the historian should ask how these groups are constructed and performed as part of their historical context. In short, "the Germans" e.g. do not exist as a continuous group throughout history because what it means for the individual to be German is contingent upon the historical context we are looking at.