Threatened by the Prussians, Duke Konrad of Mazovia invited the Order of the Teutonic Knights in to help defend against them.
The Order of the Teutonic Knights was a short lived organization. 
mnli hypothesis: The Order of the Teutonic Knights was a short lived organization.  premise: Threatened by the Prussians, Duke Konrad of Mazovia invited the Order of the Teutonic Knights in to help defend against them.