Complex sentence: Not all Soviet military commanders recognized the T-26 light infantry tank's obsolescence in the mid- 1930s and work designing tanks with antiprojectile armour was slow in the USSR at that time.
Split into simpler sentences: Not all Soviet military commanders recognized the T-26 light infantry tank's obsolescence in the mid- 1930s. Work to design tanks with antiprojectile armour was slow in the USSR at that time.