This article is not about the European Union.
Much has been written about the dangers of integrating software components late in a project. "Late" could mean "not at the first opportunity", "not after every working change to every module" or "closer to the product's deployment date than the project's commencement date". Any definition suffices for the purpose of this article.
One reason that exacerbates late integration is that not all the software systems that need to be integrated with the system-under-development (SUD) are known in advance. This problem is (in software terms) ancient, and apparently bit the first XP project -- "end to end is farther than you think". 
It is always going to be difficult to a priori enumerate the complete list of software systems with which a given SUD is going to integrate. And such a list would be an evolving artifact, anyway. So how do we continually determine with which systems our SUD is expected to integrate?
One way is to make this apparent by representation. Most iterative teams regularly present their evolving product to the customers and stakeholders -- usually at the end of each iteration. This is a natural junction where integration points with other systems can be demonstrated.
How do we know if we're missing some key integration point(s)? Expand the membership of the "iteration showcase" meeting to representatives of external system who might have an integration point. It may seem chaotic to invite too many people to an iteration showcase, but in practice this works out well. Quite often, such "chickens" either quickly transmogrify into "pigs" or leave the roost.
The foil to this wishy-washy "come if you're interested, don't if you aren't" is the statement "you probably are interested if you use an application that consumes data from or provides data to this SUD". I like to condense this to the pithy "no integration without representation".
Explicitly proclaiming that the SUD will integrate with all (and only) the applications whose stakeholders are represented has a self-correcting effect on guiding the integration points. Of course, encouraging, enabling attendance, and welcoming the representatives who might have an integration point are all necessary to the correct usage of this pith.
 See the "Creation Story" in Extreme Programming Explained by Kent Beck.