The theory — which has evolved since pioneering female peacekeepers started participating in U.N. missions in the Balkans in the 1990s — is that women employ distinctive social skills in a rugged macho domain. They are being counted on to bring calm to the streets and the barracks, acting as public servants instead of invaders.

“When female soldiers are present, the situation is closer to real life, and as a result the men tend to behave,” said Gerard J. DeGroot, a history professor at the University of St. Andrews in Scotland who has written books about women in the military. “Any conflict where you have an all-male army, it’s like a holiday from reality. If you inject women into that situation, they do have a civilizing effect.”

As modern peacekeeping has evolved into nation building, the number of female police officers in U.N. peacekeeping missions around the world has doubled during the past five years to 833, or more than 6 percent of a force of 12,867. Nigeria and India are top contributors to a total that is still far below the international goal of 20 percent. In some missions — notably Darfur and Liberia — women are edging closer: Women account for 14 percent of the 1,354 police peacekeepers in Liberia.

Liberia — a West African country created in 1847 to settle freed American slaves — is something of a modern laboratory for the rise of women making peace. Women are marching in foot patrols; the head of the U.N. mission, Ellen Margrethe Loj of Denmark, is a woman; and the Liberian president, Mrs. Sirleaf, is the first woman elected as an African head of state, in 2005.

Mrs. Sirleaf — whose nickname is “Iron Lady” — is particularly blunt about the role of women in the recovery of her fragile country, which was battered by 14 years of civil war that left about 200,000 people dead and survivors haunted by torture, systematic rapes and the exploitation of drug-addicted boy soldiers.