If the US attacks Iran, that could mean a world war, the US against Russia and China.

One important point of dissimilarity between the present is that, in 1914, none of the European powers had a way to mobilize only part of the army. Mobilization was a complex plan, and the only plan was for complete mobilization. This meant that a country which feared attack had to mobilize completely, which meant its other enemies had to do likewise.

If a country which had feared an immediate attack had had the ability to mobilize partially, that might have deterred its enemies while not threatening them.

There is no such problem today, with mobilizing reserves. But I don't know whether that makes a big difference.