Why didn't Saruman kill the Ents himself?

[Smaug The Golden]

Recently I've been thinking about Saruman's military strategy during the War of The Ring. I've been pondering certain things about it and want to hear some other's thoughts.


First question, why did Saruman attack Rohan so prematurely with his army? Would it not have made more sense to wait until Sauron attacked Minas Tirith, and then attack Rohan in a joint assault? Neither nation would be able to reinforce the other, and most likely both battles would've been won by the aggressors.



Another question I was thinking of was, why did Saruman not fight the Ents himself when they attacked Isengard? Saruman being a Maiar had great power, though it was restricted by being an Istari. But regardless, would he have been able to fight off the Ents attacking his fortress? Gandalf fought Durin's Bane on equal footing (and we know Balrogs are far more powerful than Ents) so I'm curious why Saruman did not leave Orthanc to destroy the attacking Ents.


Would like to hear other's insight.