The populations of some countries like Czechoslovakia and
Yugoslavia were thrown together by the Allied forces who "mopped up"
World War II. What problems are we seeing come alive now, as a result
of forcing certain populations to live too near each other?

need a one page paper on the above information with links where info came from

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If this is a homework question, I hope it is worded differently to the
question you've asked. First, both Yugoslavia and Czechoslovakia
existed as nations before WW2, with much the same boundaries as after
that war - in fact, both came into existence just after WW1, with the
collapse of the Austro-Hungarian empire. Second, the phrase "Allied
forces" usually refers to the Western allies (UK, US etc), whereas
both Yugoslavia and Czechoslovakia were liberated from Nazi occupation
by the Soviet army, and then came within the Eastern bloc until the
end of the Cold War - though Yugoslavia, led by Tito, broke away from
the Soviet Union at an early stage.

Further to geof-ga's point

Much of that area was administered by the Turkish Empire who took over
Constantinople around 1400

The Turks were pretty keen on ensuring that people were Moslem, as a
result some converted and some did not, but those in the same
geographical area were really the same people regardless of 'religion'
or what they called themselves.

Sure, on the borders, like Kosovo and Montenegro, you get Albanians,
but that is just on the border.

What actually happened post WWI was that large empires were carved up
into smaller states, which persisted until recently, when they started
breaking into even smaller states.

With very few exceptions, there are not many cases 'of forcing certain
populations to live too near each other'

The populations are just there, occasionally there are inappropriate
boundaries, Africa was carved up peculiarly, without much regard to
tribes, but that was inevitable.
Look at racial classification in Rwanda 
- the Tutsis and the Hutus were separated as 'races' when issued ID cards.

http://www.pbs.org/newshour/bb/africa/jan-june99/hutus_tutsis.html

It would probably be better to look at areas where the 'tectonic
plates' of populations rub against each other
- the Balkans is a classic, Turk, Teuton and Slav
- Turkey and Armenia is an unpleasant example
- The Northern Alliance in Italy is interesting

Another is Sudan, yet another (possibly) Cyprus

There are instances of 'transplantation', the best ones look like
immigration in say the UK and Russian colonies in the former USSR
- yet again, all was largely voluntary

Go for the 'tectonic plates' angle
- if not that, then enforced migration