Infantry can attack an enemy territory either with or without armor, fighters or other units to support them.
And yes, it is a favorite strategy to do exactly that. 2 Attacking Infantry vs 1 Defending Infantry has a 1.41:1 odds of success. So you can imagine 20 infantry vs 10 infantry would be an even better chance.
In other words, buy infantry, buy lots of infantry. Use them as defenders and fodder for your main attacks, but don’t dimiss their ability to fight on their own either. In small battles a couple of infantry is just as good as tanks or planes.
I hope that helps. I didn’t just want to post a “yes” to your question.