At an army checkpoint near the occupied city of Donetsk in eastern Ukraine, Lt. Col. Natalia Semeniuk approached a convoy of two minibuses that had just arrived from Kiev. Slung over her shoulder was an AK-47 assault rifle. Clumps of long, brown hair poked out from beneath the beanie she wore to guard against the cold, which had dropped to about minus-20 degrees Fahrenheit.

Colonel Semeniuk was meeting with Anna Sandalova, a former public relations executive and a founder of Help the Army of Ukraine, a foundation that uses Facebook to raise money to buy equipment for Ukraine’s desperately underfunded army. Ms. Sandalova has become something of a star in her country, especially to its soldiers. She supplies them with everything from body armor to night-vision goggles, to sleeping bags and food. Since her group was established in March, it has raised over $1.3 million, Ms. Sandalova told me, for the fight against the pro-Russian separatists who have occupied large parts of eastern Ukraine.

An overwhelming majority of the money is crowdfunded from the Ukrainian people through Facebook. The process is simple: Ms. Sandalova liaises with army divisions fighting in the field. They tell her what they need and she posts their requests to Facebook. People donate via bank transfer into the foundation’s account, and Ms. Sandalova and her colleagues then drive the goods to eastern Ukraine personally.

That day, the minibuses were filled to bursting. Clad in body armor and a helmet, Ms. Sandalova followed Colonel Semeniuk to a Ukrainian Army camp in the forest near Donetsk to make her first delivery. Canvas tents dotted the area, erected among thickets of trees covered with snow. Soldiers huddled together, talking and smoking. Some helped unload several mobile shower units.