Advanced armaments need rare resources like oil and rubber.   Rare resource is limited in map, thus you have to be strong enough to compete for rare resources and defend your own. 

There are several ways to obtain oil and rubber:
1.Build mini rubber factory and mini oil field in the city and upgrade them to raise production.  
2.Capture resource zone that labeled with oil can or tire. 
3.Upgrade your commercial center and trade for oil and rubber. 
4.Complete battlefield missions to get them.