Complex sentence: French 75 shells, at least initially in 1914, were anti-personnel projectiles: they had been designed to inflict maximum prejudice against enemy troops stationing or advancing in the open.
Split into simpler sentences: French 75 shells, at least initially in 1914, basically were anti-personnel projectiles. They had been designed for the specific purpose of inflicting maximum prejudice against enemy troops stationing or advancing in the open.