To fight or to wait, that was the critical choice laid before Erwin, but it was a dilemma that didn't confound him for long. After a brief hesitation, he made his decision immediately.
Fight!
He absolutely couldn't give Rocky any chances!
Although Rick had clearly informed Erwin that reinforcements would certainly arrive in half a month, theoretically, all Erwin had to do was to hold out for that half-month until the good times rolled in. It sounded fine in theory, and the time wasn't perceived to be very long, but only Erwin, who stood on the front line, knew that it was practically impossible.
In half a month's time, who knew what new form of advanced technology Rocky could come up with? If Rocky introduced some new technology during this period or even if he merely expanded his current forces, Erwin wouldn't be able to wait for the reinforcements—he would be wiped out by Rocky directly. Hence, Erwin was resolute in not giving Rocky any opportunity whatsoever.