Complex sentence: The period was also marked by an arms race that primarily affected the artillery, with improvements in heavy artillery (Guns of 120mm and 155mm, Models 1890 Baquet, new hydropneumatic brakes), but also and especially the development of the ultra secret 75mm gun.
Split into simpler sentences: The period was also marked by an arms race that primarily affected the artillery. There were improvements in heavy artillery (Guns of 120mm and 155mm, Models 1890 Baquet, new hydropneumatic brakes), but also and especially the development of the ultra secret 75mm gun.