Global oil prices have fallen 25 percent since June, marking the return of oil-price fluctuation as a geopolitical wild card. Although oil prices have been relatively stable — about $100 a barrel — for the past five years, the historical pattern has seen high oil prices boost the strategic clout of producing countries by boosting government coffers, and falling prices have had the opposite effect.

The lower prices of the 1980s and ’90s, for example, weakened Russia’s geopolitical position, while the more recent increases have boosted Moscow’s ability to exert influence over former Soviet republics. Higher prices have enabled Venezuela’s leadership to strengthen opposition to U.S. influence throughout Latin America.

Fluctuations have had significant effects on economic growth. The fourfold increase imposed by the Organization of Petroleum Exporting Countries in 1973 to protest U.S. support for Israel during its war with Egypt and Syria that year boosted the revenues of producing countries while retarding economic growth elsewhere. Subsequent price drops slowed the economies of the producing countries while consumers elsewhere enjoyed an invigorating lift.

Lower prices tend to discourage investment in new projects, especially those involving large startup costs, such as deep-water and Arctic operations, thereby affecting long-term petroleum supply. The lower prices of the 1980s and ’90s caused a significant dip in major project development, resulting in supply shortfalls in the early 2000s and a price surge in 2005 through 2008. High prices, on the other hand, invite investment in such projects, resulting in the expansion of global supply.

The average $100 per barrel of recent years has encouraged the major energy companies to invest heavily in new projects, focusing in particular in such unconventional sources as Canada’s tar sands, U.S. shale, the offshore ocean floor and the Arctic. The new supply streams of brought online by these investments eased fears of peak oil and declining world output, and they shifted the center of gravity of world oil production from traditional suppliers in Africa and the Middle East to the shale and bitumen fields of North America.

But this surge in global production has collided with an unexpected slowing of global demand as a result of Europe’s economic downturn and decelerating growth in China. On Oct. 14, the International Energy Agency lowered its 2014 estimate of world demand by 200,000 barrels per day and its 2015 estimate by 300,000 barrels. Nevertheless, it said global supply was continuing to expand — the result of increased production in North America and the resumption of exports by Libya. The result, not surprisingly, has been a dive in prices.

This decline has altered the economic and geopolitical calculations of major producing countries, now forced to plan for decidedly lower returns from their oil exports. Falling oil prices will prove especially troubling for producers like Iran and Russia that rely on petroleum exports for the lion’s share of government revenue. Budgets in both countries are premised on $100-plus oil, and the falloff in state revenue will curtail international endeavors. Russia, for example, has been forced to reconsider plans for an expensive overhaul of its armed forces.

While every major producer is likely to suffer as a result of the price decline, those with lower production costs and budgetary expectations are expected to fare better than those with higher costs and expectations. Saudi Arabia, for example, is better positioned to weather the downturn because it enjoys relatively low production costs and can still finance most of its government obligations.

Some analysts speculate that there’s a geopolitical motive behind the Saudis’ rebuffing calls for OPEC to reduce the production quotas the cartel has traditionally used to keep prices higher. That’s because the lower prices are less harmful to Saudi Arabia than they are to Iran and Russia — countries that have angered Riyadh by helping prop up Bashar al-Assad’s regime in Syria and, in Iran’s case, for contesting Saudi influence throughout the Middle East.

Although the United States has emerged as a major producer, the impact of falling oil prices is, on balance, a positive one: While some energy firms in the U.S. could suffer from falling prices, the wider economy has benefited as lower energy costs boost an anemic economic recovery. The oil-price pressure on Russia bolsters U.S. efforts to change Moscow’s calculations in Ukraine by imposing economic sanctions.

The extent of these geopolitical shifts will depend on the duration of the price decline. Should oil prices stabilize in the coming weeks, the consequences could prove less than momentous; a bigger drop and longer decline would have farther-reaching effects. Whatever the outcome, the current dip has exposed weaknesses in the global oil enterprise that are not likely to disappear soon, even if prices rebound.

Ever since recovering from the financial crisis of 2008 and ’09, the major Western oil companies have based their long-term strategies on two key assumptions: First, that continuing economic growth in Asia and other emerging markets will generate an unquenchable thirst for petroleum products and, second, that mammoth investment in unconventional oil projects will be redeemed by unflagging increases in demand. Both assumptions now appear seriously flawed.

China’s growth has begun to sag, falling to 7.6 percent in the second quarter of 2014 — the lowest since the financial crisis and a marked retreat from the 9.5 percent of one year ago. President Xi Jinping has promised a vigorous drive to reform the economy and stimulate growth, but few analysts expect any significant results in the next few years. Exuberant growth in Africa and a few other developing areas may help compensate for the sluggishness in Asia, but is unlikely to generate the ever-expanding demand once anticipated by oil company strategists.

The other pillar of the oil industry strategy — ample returns on costly unconventional projects — is showing signs of strain. Endeavors such as drilling in the Arctic and exploiting Canada’s tar sands are profitable only at prices of $90 a barrel or higher. The extraction of oil from shale might remain profitable with prices hovering above $80, but some high-cost producers could be forced to suspend operations. And if prices fall much below $80, the prospects for many unconventional projects could prove dismal.

Should prices climb back above $100 per barrel, many of these undertakings could prove attractive again. But higher prices are likely to inhibit economic growth — and without growth, demand may not rebound. Low-cost producers like Saudi Arabia might prosper in such an environment, but high-cost producers like Canada and the U.S. may not fare so well.

Under these circumstances, the major oil companies may conclude that significant alterations are needed in their long-term strategies for success. What form those could take is not immediately evident, but it is this prospect, more than any other, that is likely to prove the most lasting consequence of the current dip in prices.