How do I socialize a chihuahua puppy who is extremely territorial?

She is just about 6 months now, and she’s the sweetest little thing, very loving and affectionate… to people. Over the past few months she’s been introduced to several different types of dogs, and it hasn’t gone well. I have a 12 year old poodle and she is perfect with her; I think it’s because the poodle was here first? But the other dogs she will attack. She wants to see them, but when they get close to her she shows her teeth and snarls and will even snap at them. Or she just hides behind me. She seems to be OK with other chihuahuas (and my poodle) but how do I get her to play nice with other dogs?