There’s been a lot of talk since the 2016 U.S. presidential campaign about how political rhetoric is too harsh. And since last month’s neo-Nazi rally in Charlottesville, President Trump and others have been widely criticized for failing to denounce extremists sharply enough. These problems are linked. Both are aspects of a single idea—preserving community.Communities are under political stress throughout the democratic world. One of us is American and one Israeli, and in each of our countries politics has grown polarized. And, in recent weeks, each country has had to deal with violence committed by domestic extremists. The neo-Nazi rally in Charlottesville resulted in the murder of an anti-Nazi demonstrator. And a few weeks before that, three Arab citizens of Israel fatally shot two Israeli policemen in a terrorist attack on Jerusalem’s Temple Mount.Community, not just land and people, is the essence of a country. It’s a type of fellow feeling, an extension of the affection people tend to have for their family members. It was handsomely on display in the mutual support shown during the recent hurricanes in Texas and Florida. A country’s key community is the mass of people that share its ideals – those who believe in its basic national and political principles and are committed to defending them.Community is the source of popular morale, so it plays a vital role in national defense. But it serves that purpose only when people in general keep it in mind, value it highly and delineate clearly who is in and who is out.National communities in both the United States and Israel would benefit from changing the way they talk about politics. First, if people stopped characterizing ordinary political opponents as enemies, it would reduce polarization. And second, it would improve hygiene in the body politic if people unequivocally labeled actual enemies as enemies. What’s called for, in other words, is more tolerance and broadmindedness regarding members of our national community, and less tolerance and stronger repudiation of the community’s domestic enemies.What do we mean by political polarization? It’s the mentality of us versus them. It’s when there’s no soothing consciousness of common ties as exists within a proper family. There’s no sense that we’re all in this together even though we disagree.Hateful language causes polarization—and also results from it. Extremist rhetoric frames issues starkly and unfairly, in ways that make it impossible for opponents to respect each other’s positions. While all decent citizens want to help the poor, people on the left and right disagree about how. It’s polarizing to paint all conservatives as inhumane or all liberals as mindless bleeding hearts. Good people all want the country to be safe, but so-called doves and hawks differ on how to accomplish this. When doves call themselves “the peace camp,” they effectively smear their rivals as opponents of peace, and it’s likewise a smear when hawks loosely brand doves as “unpatriotic.”Advocates of a given policy will often suggest that the only people who oppose them are racists, fanatics, traitors, idiots and/or scoundrels. But, for ordinary matters, that is just about never true. A proper citizen won’t engage in—let alone reward— divisive politics of this kind, however effective it is and however valid the policy argument it is trying to advance. The foundation of democracy is the recognition that reasonable people can differ on policy. Polarizing talk is harmful because it wrongfully banishes people from the national community, which weakens the country.At the same time, some citizens are in fact hostile to the country’s national and philosophical principles. In their case, it’s damaging if we fail to exclude them from the community. That’s why it’s important for political and other community leaders to repudiate domestic neo-Nazis, terrorists, democracy-haters, and other outright enemies.Shunning extremists is not a complete solution to the challenges they pose as disloyal citizens, but it can help. People who actively hate their own country often have cherished attachments to subgroups in society—to family members, coreligionists or circles of friends—that include loyal citizens (or, at least, citizens who are not active enemies). The latter can be influential if they take a stand for the community at large and punish disloyalty. If, on the contrary, they shelter and make excuses for the bad actors—or, even worse, honor them—they are encouraging evil.So, when young extremists commit political violence in the name, for example, of the Muslims, the Jews, or the socialist class struggle, it’s crucial that they be denounced and rejected specifically by those fellow Muslims, Jews, or socialists who want to retain good standing in the national community. The extremists themselves won’t care about such standing, but they may care deeply about being despised by and excluded from their particular subgroup.In democracies like America and Israel, consciousness of the national community and serious thought about who is in and who is out are vital to political well-being and to defense against enemies foreign and domestic. A big-hearted community that clearly stands for something works for us as both a shield and a sword.