The treaty of Versailles virtually guaranteed that there would be a second world war. In fact, many historians believe that WWI never truly ended, and that it just had a 20 year break until Germany invaded Poland in 1939.
The Treaty of Versailles is also indirectly responsible for why we invaded Iraq as well. By granting border rights to colonial powers after WWI, the British divided up parts of the middle east into separate nation states, and this is why Hussein went into Kuwait in '91. And we all loved him prior to that so its safe to say that without that invasion in 1991, there would have been zero pretext for this most recent bloodbath in Iraq.
That being said, foreign policy can have far reaching consequences and severely consequential unintended results, so we should be really, really careful about it.
The Marshall Plan was successful and cheap, but it worked not because of only the money we spent, but because we were working with western countries with similar cultures to ours. The Marshall plan has been described here as an example of why we should continue to send over young men and women to die for nothing in Iraq. But Iraq is not Europe, and nothing will make them become our allies in any meaningful sense.