The Polish Prime Minister Jaroslaw Kaczynski got the meeting of European leaders off to an acrimonious start by complaining that Poland would have a far bigger population on which to base its EU voting power if it had not been for the German invasion in September 1939 - the start of the Second World War.
It's true that WW2 cost them about 20% of the population. I think we should remember that as the Wehrmacht swept East, held up only by the heroic flank attack of the Bzura, the former Soviet Union also invaded, accounting for a fair few of those victims themselves.
Given that a million or so of the survivors descendants are now in the UK, they'd have a far bigger population if they'd not joined the EU, too.
UPDATE - Martin Kelly remembers an earlier conflict in that neck of the woods and has some wise words on integration, noting how commanders of Scottish descent served on both sides. Our WW2 army was full of generals with names like de Guingand and Carton de Wiart, Pierre de Villiers, LeQuesne Martel, Gott and Wernher.