Complex sentence: The US had tried but failed to stop or obstruct the prosecutions by Italy's independent judiciary, so instead they used threats and diplomatic pressure to prevent the Italian government from forwarding extradition papers for the convicted US agents.
Split into simpler sentences: The US had tried but failed to obstruct the prosecutions by Italy's independent Judiciary. Following the convictions the US used threats and diplomatic pressure to stop the Italian Executive branch from issuing arrest warrants and extradition requests for the convicted US agents.