Inability of a long-term bond market intervention to contribute to achieving a peg In «The Federal Reserve's Large-scale Asset Purchase Programmes: Rationale and Effects» 2012 paper, there's the following sentence:


  According  to  the  pure  expectations theory of the term structure,
  targeting three separate interest rates should succeed if and only if
  the private sector's expectations of the path of the short rate are
  aligned in a way that precisely   generates   the   targeted
  configuration   of   rates. In this environment,  for  a  given 
  expected  path  for  the  short-term  rate,  direct  intervention in 
  longer  maturity  Treasury  markets  could  not  contribute  to 
  achieving  the  peg.


Why is it that intervening in the long-term market would not help achieve the peg? 

Any help would be appreciated.
The issue isn't so much that intervening in the long-term market wouldn't help, but rather that under rational expectations all the return rates on government bonds are determined by the same process, and thus have to be consistent with each other and converge to the same rates. What D'Amico et al. seem to be suggesting is that the Federal Reserve tried to enforce mutually inconsistent rates on bonds with different maturities, such that there was a sequence of operations on bonds with short-term maturities (what I think they're calling "an expected path for the short-term rate") which would yield a return either above or below that of bonds with different maturities.

What it seems that they're really saying is that, under rational expectations, the FED would have had really just one policy instrument: the interest rate at any time. Bonds with different maturities just had to conform to that rate. But in practice, the FED tried to enforce a set of different rates, one each for each kind of maturity, which the authors think it's doable because the variance of bond prices is lower than what would be expected under rational expectations (hence the "the slightest fluctuation in expectations of the short-rate path would immediately imperil the peg" bit).