6-month Boxer with food agression

Question
My female boxer is 6 1/2 months old.  We got her at 8 weeks.  I've had Boxers (she's my 4th), so I have some experience with the breed but never encountered this before.

In the house is an 8-year old Beagle, and my Boxer.  The Beagle is a rescue and will eat anything in sight.  From the beginning I started feeding them separately, at the same time but the Beagle eats outside the patio door, and the Boxer inside, the door is glass, they can see each other.  I have had no problems until the past week.  My Boxer, on a daily basis, has become agressive and attacking the Beagle.  Not when eating, because they are separate.  But if the Beagle just sniffs the floor, she goes for her.  Any suggestions?  My prior Boxers were so docile, anyone or dog could approach the bowl or treat without an issue.

The Boxer came from a rescue.  Her mom was rescued while pregnant. We followed her through her pregnacy, and got to meet her.  We never met the father.  The mom was about 1 year, her first litter, of 3.  My Boxer is beautiful, beautiful fawn color with striking white flash on her chest and paws.  Her jaw lines up nicely, no overbite.  She does not look like she is mixed with anything but Boxer.

Answer
It sounds like the Boxer is coming into her own and developing an alpha attitude.

You need to do three things.

First, buy a whistle. Each time she misbehaves, blow the whistle and it will startle her. When she stops, push her down to the floor and hold her in a submissive position and do not let her up until she submits.

When it is feeding time, place her dog dish on the counter or table with food in it; place cookies or crackers on the table behind the dog dish. Have her sit before you. Stand and eat the cookies or crackers without looking at her, one by one. She will think you are eating out of her dog dish.

When finished, place her bowl down in front of her and give her the verbal command to eat. Do this each meal for a month.

In the wild, the pack leader-- the alpha, eats first while the pack waits and watches. When the alpha is finished, he allows the pack to eat what is left. So it is dominance.

Once she know you are the alpha, she will begin obeying. Alphas also make the subordinate submit to them by putting them down and holding them until they submit. That is how the alpha disciplines.

Try these and let me know how she is in a few weeks with a follow-up or by email.