The weather was very cold and the supplies were dwindling.
In his poem "Saint Filomena," Henry Wadsworth Longfellow describes the terrible conditions that Florence Nightingale found when she and her fellow nurses attended to patients during the Crimean War. She held the position of superintendent of the Female Nursing Establishment of the English General Hospitals while she was in Turkey. While she was there, she made large changes to the conditions that injured men were placed in at the hospitals.
The conditions Nightingale found in the hospitals were horrifying. People were injured, packed in rooms together, and dying of disease in their own filth. When Longfellow writes of the cold, damp trenches and the frozen, starving camps, he's showing the kinds of conditions that the patients faced before they came to the hospitals where things were just as bad. Without Nightingale's support and fierce determination, many more men would have died in squalor.
Wordsworth explains that he is reading about the war: the frozen weather and the low supplies, the injured soldiers and the terrible hospitals they ended up in. But there's a spot of hope—the lady with the lamp lights up the place and gives them a reason to think things might get better. He's referring to Florence Nightingale and using the horrid conditions as a way to show how bad things were before she arrived and changed them.