Roman legions never attacked India. It was too far. Historically Roman legions were only defeated twice- In cannae by hannibal of carthage which led to drastic reorganization in army and enabled romans to wipe carthage from face of the earth in their counterattack. The second by Germans and Romans never counterattacked as Germany was deemed too cold to hold as a garrison. The Roman soldier was never famous for valour but tactically romans adopted simple but very effective tricks. Infact our current conflict with Pakistan and Islam can be assessed in light of the tactics which Rome adopted when facing challenge from Carthage. These were:
a) Focus on effective body armour (Today it is bullet proof jacket) . Romans had the best body armour of their time
b) Change from defensive to offensive tactic. Romans realized that a small force on offense is much more effective than large force on defensive
c) Utilizing proxies to weaken their adversaries before going to war
d) Out of turn promotions for soldiers and generals who showed leadership in war. Romans were never sticklers for hierarchies in war
e) Finally Romans believed in quick adoption of weapons technology through reverse engineering followed by improvization instead of waiting for ideal weapon.
f) Showing absolutely no mercy to their enemies-civilians and soldiers alike
Contrast this with our philosophy which gave emphasis on peaceful negotiation, forgiving enemies, preferring defence over offence. We were never short of valour but lost out always on tactics and leadership.