What precipitated the rise of social safety nets in the United States?

Of course the main reason was the Russian Revolution which caused huge changes in social standards throughout all the Europe, and to a lesser extent, in America.

If you look back through history, the root cause of major social reforms in the United States was the Great Depression of 1929. Prior to that, most Americans were self-sufficient and fiercely proud. The Great Depression changed all of that practically over night.

There were a number of other factors that were already beginning to develop, most notably the failure of a number of businesses just prior to the stock market collapse in 1929. However, the actual collapse set into motion a domino effect that would be felt throughout the entire world. Several banks failed, resulting in millions of people losing their entire life's savings. Many factories closed, resulting in lost jobs, and a number of farms went into foreclosure, resulting in a decrease in the production of food. Local governments started facing collapses because they couldn't raise any tax revenue to keep their own programs going due to the fact that so many people no longer had any money.

To make matters worse, the government voted to hike tarrifs, resulting in other countries responding by raising their own tariffs. Suddenly nobody wanted to buy American goods, and Americans couldn't afford to buy foreign goods. This ended up resulting in a world-wide depression. In 1930, over 4 million people were out of jobs, and by 1931, that number had doubled. As if this wasn't bad enough, the Midwest experienced a devastating drought about the same time, resulting in even more farms failing and a number of people being forced to migrate to other states looking for work, placing an even greater burden on those local government.

When FDR was elected, he initiated a number of government programs focused on relief, recovery, and reform. Since Congress was controlled by the Democrats, he had no resistance in enacting his programs. New welfare programs were created, Social Security was introduced, and a number of job programs were initiated. All of these things combined helped to poise the United States for a major recovery, which happened to coincide nicely with the outbreak of World War II. The United States was just beginning to redevelop a strong workforce, and they had a number of countries suddenly wanting to buy their goods.

After or during World War II, many of these programs were dissolved, but many others ended up growing into the social welfare programs we have in such great abundance today. Sadly, the job creation programs began to disappear, but the "handout" programs remained. Over time, they have become so ingrained in our country, it would probably be next to impossible for any form of government to ever eliminate it. As a result, the need or desire to be as self-sufficient as their ancestors has bgan to dissipate with subsequent generations.

From at least the Middle Ages, there had been a combination of systems to support the poor, the old, the infirm, and so on. People lived in networks of extended family, and the more successful members of a family were expected to support the less successful; that was the first stage of the safety net. Once family resources were exhausted, or if you had no family, the next stage of support was the Parish - every Parish was expected to support the local poor (at this stage it is very hard to distinguish between Parish as religious organisation and Parish as state organisation, due to the intermingling of Church and State and the fact that those appointed to the role are often also church wardens or similar). There are also alms-houses set up by philanthropists for some few poor people, and in later times there were efforts to look after sectional groups by for instance the institution of military pensions in the late 1600s. None of these systems were perfect, and many people who were unable to work were left to starve - or turn to begging or thievery to sustain themselves.

Then along came the Industrial Revolution; and where most people used to live amongst their extended family, suddenly there were huge populations with very few family nearby. Anyone can be unlucky - but how much more so when people are working 12 or 15 hour shifts in factories filled with dangerous machinery. Deaths were common, injuries much more so, and the old systems were overwhelmed. Bear in mind that this is all in the UK, where the alterations were less profound than in the US, because in the UK at least the rural areas were remaining relatively unchanged, and the old systems were well-established. In North America, even the rural areas were often newly settled, and family could be thousands of miles away instead of merely dozens or hundreds. Additionally, people living in urban areas cannot fall back on growing all their own food in times of hardship; they are more likely to be renting accommodation than to own their own land, and so on. So in towns the problem of poverty is more concentrated, and it is more severe. If the family has a breadwinner who can find employment, they are likely to be okay. However if there aren't enough jobs (e.g. Great Depression), or you're in a group that finds it difficult to find employment (e.g. elderly, child, woman pre-1970-ish), you are going to be reliant on support from somewhere. Logically, like most other things in modern Western societies, the x-of last resort is the Government. When you can't put out the fire in your house on your own, you call the fire brigade. When you can't educate your children yourself you send them to school (/apprentice them off in earlier times). When the banks are on the brink of collapse, the government steps in to guarantee them. And when other social safety nets are inadequate, the government steps in. Certainly the experience in the UK was that it was often the groups and organisations, charitable, religious or otherwise engaged in trying to alleviate social problems on the ground who wanted the government to get involved, because the problem was too big for them to solve on their own. Look at the cases of Joseph Rowntree, Charles Booth and the Cadbury family for people who started in industry, tried to look after their workers, and then moved into wider philanthropy and arguing for government intervention. An American equivalent is Milton Hershey. This experience is common in many ways across the industrialised world; as society shifts, and old systems break down, new solutions must be found to age-old problems; everywhere the problem of how to support those members of society who cannot support themselves has included the involvement of the government to a greater or lesser extent.